Critical reception
Screen Rant's Spencer Coriaty wrote: "Backed by some of the most cutting-edge animation at the time, and still breathtaking by today's standards, Invader Zim is like a Mystery Science Theater fan's dream come true. It seamlessly blends sci-fi, action, and comedy into one twisted and hilarious cartoon with superb voice acting and visionary direction." Kristy Punchko from Nerdist praised the series' satirical themes and how it "relished in weird, pushed boundaries, and dared to be unapologetically stupid". Punchko also offered praise for the humor in Invader Zim, saying: "Colliding the sublimely silly with the cerebral, Invader Zim played as seminal precursor to subversive sci-fi shows like Rick and Morty." Becca James, writing for The A.V. Club said, "Invader Zim was destined for a cult following. From its seemingly bizarre nature to its too-early demise, [Invader Zim] brought a darker form of entertainment to [Nickelodeon and] welcomed critical acclaim for straddling the line between child and adult entertainment [and the] constant depiction of Earth as a complete [expletive] only adds to this dingy, but delightful program."
Allegra Frank from Polygon said: [The series is laid out] in a story-focused half-hour unlike anything ever aired on Nickelodeon previously. Invader Zim had a villain at its center, years before Breaking Bad and Mad Men popularized that dramatic convention for adult audiences. As a character, Zim was mostly unlikable; the show's color palette was narrow and unfriendly, and the entire design of the series — interiors covered in grime, streets covered in garbage, on-screen televisions full of screaming ads — dared viewers to be turned off. While jokes could easily revolve around the high-pitched screams of Zim's useless robot companion, GIR, the comedy was also often derived from much darker places, like harvesting organs from living children and lice outbreaks and characters slowly turning into a sentient [bologna]. Dib wanted to kill Zim, after all; that's not usually the subject of children's TV. Sam Thielman from The Guardian said, "[Invader Zim]'s sense of humor is somehow dark without being bleak; even when the show takes its cues from horror movies it's often quite scary, but it's somehow never too much. [...] Invader Zim didn't last nearly long enough, but it did spawn a cult following almost immediately and became a password for alienated millennials and teenagers everywhere." Alex Bedder, from NYU Local, said, "Essentially the black sheep of the animated Nickelodeon family, Invader Zim never quite stuck its landing, but it became a beloved and off-kilter fan favorite. [...] There are random, twitchy characters, the word 'doom' is overused, and there are countless other illogical outbursts. But the common thread weaving all of the madness together is its original, sick, and sometimes oddly-intelligent sense of humor." In an article he wrote for Cartoon Brew, animation historian Jerry Beck said: "I always thought highly of [Invader Zim], but watching it again this weekend reminded me how good it truly was. [...] I laugh long and hard at incidents, situations and visuals on [Invader Zim], and if there is any justice it should be ranked alongside the likes of South Park and The Simpsons."
Mary Grace Garis from Bustle praised Invader Zim for how it does not sugarcoat reality, saying, "You can't deny that [Invader Zim] really put in some heavy, soul-crushing messages between 'The Doom Song' and all those rubber piggies. And I'd say that à la The Powerpuff Girls, they did a really good job of hiding adult lessons in a children's cartoon, but well... I'm pretty sure they were a bit more blatant in delivering the news that we were all doomed." Kayla Cobb of Decider said, "If you prefer your humor to be equal parts dark and random, then it's time to re-watch this short-lived Nicktoon. [Invader Zim] can best be characterized by its sarcasm, cruelty, and violence. This is a show that creates a disturbing giant-monster-baby fusion in one episode and labels the ultimate face of horror as a room with a moose the next. So yeah. It's kind of all over the place, which is why we love it." Lana Berkowitz from the Houston Chronicle praised Invader Zim's art-style and color palette saying, "The series has an appropriately gloomy look of impending doom colored with dark reds and black, and a dash of alien green. The animation's angular look and the characters' big, expressive eyes give Invader Zim a distinctive look."
Variety's Steven Oxman wrote, "[Invader Zim] captures a nice blend of the innocent and the satirical", further stating that "the artwork [in the show] isn't especially original but still manages to create a nicely off-center vision of Earth and its inhabitants. Vasquez and co-writer Rob Hummel throw in some clever quips at life on this planet, taking aim particularly at the doomsaying schoolteacher, Ms. Bitters, [and] Kevin Manthei's music provides strong accompaniment to the escapades." Sean Fitz-Gerald from Thrillist wrote: "Though Invader Zim came from Jhonen Vasquez, the same beautifully twisted mind behind Johnny the Homicidal Maniac, the [show] manages to hold off on graphic violence and obscenities without losing all its bite. The spastic [Zim], along with the world around him, capitalizes on cheeky, masochistic, and random humor in ways viewers of all ages can appreciate." Joe Matar, writing for Den of Geek, wrote: "Invader Zim is a brilliant piece of unsettling, grotesque horror, with a heart of goofiness, which is a big part of its appeal. [...] The incorporation of cel-shaded CGI and lots of sharp geometric shapes actually enhanced the look [of the show] rather than came off as a cheap shortcut." Andy Patrizio of IGN praised the colors and the CGI imagery in Invader Zim, but criticized how Zim is "always shrieking and yelling at the top of his lungs" and the inconsistencies between episodes, but added "When the show hits, it hits big."
George Dvorsky, from io9, specifically praised the character GIR, saying, "Science fiction has portrayed its fair share of glitchy and bumbling robots over the years, but none hold a candle to Invader Zim's GIR. Quite possibly the most erratic and unhelpful robot to ever hit the screen, GIR has become one of the most loved and often quoted characters to appear in a sci-fi cartoon in years." Lynne Heffley, writing for Los Angeles Times, said "The deliriously original Invader Zim rocks" offering praise for the show's humor and art-style, calling it a "visual feast of geometric lines, strange angles, vaulting curves, fantastic, transforming machines and odd, shadowed places." Sean Aitchison from CBR said, "Invader Zim not only holds up incredibly well, it also feels like it was rather ahead of its time and maybe it would have lasted longer today. [...] The weird humor, the eccentric characters and even the premise all feel like a show that would have easily gotten green-lit in the modern cartoon renaissance that began with Adventure Time. The show only ran for two seasons, which makes it such a shame when you realize how much it would have thrived in modern times." On the review aggregator website Rotten Tomatoes, Invader Zim currently holds a 100% approval rating based on 13 reviews. The website's critical consensus for season one reads: "The right balance of dark humor and spastic earnestness make Invader Zim a creepy kid's cartoon worthy of its cult following."
The now-defunct magazine Christian Parenting Today negatively criticized Invader Zim in their May 2001 issue, written by Jennifer Mangan, calling the show "non-Christian", "immoral", "offensive", "blasphemous", "unsuitable for Christian children" and "insulting towards Christian values and beliefs" due to the show's dark nature and negative characters like Gaz and Ms. Bitters, among other complaints. Common Sense Media's Andrea Graham wrote more negatively of Invader Zim, praising the show's "laugh-out-loud" humor, but criticizing the way that Zim has a complete lack of concern for all life, how humans are depicted as less-than-intelligent life forms, how human society is depicted as a terrible disgusting place, the very frequent use of verbal insults, and that there is no good messages or good role models in the show. Some parents have criticized Invader Zim for its pessimistic portrayal of humanity, the near constant-level of screaming among the characters, as well as the large amounts of verbal insults and threats, and the disturbing and disgusting content, calling it "unnecessary", "misanthropic", "too scary for children" and "inappropriate for children." According to director Steve Ressel, Nickelodeon held a test screening for the episode "Dark Harvest" during which one kid ran out of the room crying before the episode was over and others were visibly shaken and clearly terrified at what they saw.
In 2006, IGN ranked Invader Zim at number 22 on their list of the Top 25 Primetime Animated Series of All Time and in 2009, IGN ranked Invader Zim at number 57 on their list of the Top 100 Animated Series. In 2016, Entertainment Weekly ranked Zim as the 18th Most Memorable Nickelodeon Character. That same year, Screen Rant ranked Invader Zim at number 9 on their list of the 25 Best Nickelodeon Shows and The Guardian ranked Invader Zim as the third Best Nickelodeon Cartoon. In 2017, CBR ranked Invader Zim at number 5 on their list titled, 15 Incredible Cartoons That Were Canceled For No Good Reason. In 2018, Paste Magazine ranked Invader Zim at number 60 on their list of The 100 Best Sci-Fi Shows of All Time. In 2018, IGN included the episode "The Most Horrible X-Mas Ever" on their list titled, 10 of the Best Holiday TV Episodes Ever.