Inspector Gadget. He regularly bumbles around getting himself in trouble, being unable to put two and two together, and flat out being Too Dumb to Live while his ten-year-old niece Penny and their dog solve every case for him and lead him to the bad guys, while he takes the credit for it.
Launchpad McQuack, of DuckTales, who in many episodes, saves his friends, or even the whole world while remaining a certified idiot. He's even admitted it on more than one occasion. In the new comics, his search for a job since Darkwing Duck went out of the vigilante business is displayed by but a single panel from his interview with the Rescue Rangers. No, Launchpad, you cannot, in fact, fly the Ranger Wing.
Toby in Gormiti: The Lords of Nature Return. He's courageous, selfless and a Determinator, but also quite frivolous, impulsive and Book Dumb, and has a tendency to pull pranks at the least appropriate moments, such as during a battle. Which becomes especially apparent when compared to his one-year-younger brother Nick.
Ironically enough, Toby is the Lord of Water, and Water Gormiti — especially Carrapax — are usually depicted as calm and wise.
His 'lack of a Delta Brainwave' is the one thing that allows him to save all life in the universe from annihilation on more than one occasion.
Nibblonian: "There is but one being that can resist them. A child of destiny whose bizarre brain wave pattern makes him immune to the Brain Spawn attack. He is the hope of the universe. The fate of your world - perhaps all worlds - rests in his special mind."
Leela: "Now, when you say 'special'..."
Walter Melon. The ad for the show says "He knows no fear. He knows no danger. He knows... nothing!" And... he really doesn't. He's the ultimate mercenary: whenever a character needs to be replaced, he can replace them. And he gets the powers. (Whether he can USE them remains to be seen, he displays varying levels of ability every episode.) This being a comedy, he almost always relies on Dumb Luck.
Jorgen: (During the Fairy Baby special) There has been a disturbance in the fairy force! Someone was just about to wish for the B word! (poof in Timmy, Cosmo, and Wanda) Ah, Turner. ''Biiiig''surprise.
Jorgen: (During the second Christmas special) It's the day AFTER Christmas! Where is all this wishing coming from?! (Scanner show the magic is being pinpointed to Dimmsdale) Turner. Of course.
Mikey Simon from Kappa Mikey is prone to being this trope.
George of the Jungle, in the eponymous television series. But Tom Slick (from the same show), a parody of the original Tom Swift, fits the trope even better. He only wins his races because his True Companions aren't morons, and the villains make Dick Dastardly look rational.
Tom Slick: Fear? I don't know the meaning of fear!
Granny: He also doesn't know the meaning of 'broccoli'.
The titular character on Jimmy Two-Shoes is dumb (or at least extremely naive), yet he remains the only person Lucius can't break.
Wakfu gives us Sadlygrove, resident knight-errant and infamous Iop-head. Also puts the emphasis on the idiot bit, at least until episode 22.
Finn from Adventure Time, by his very own admission with the page quote. Though he's more impulsive than outright dumb—he occasionally wins challenges through craft, for example ("The Limit," "It Came from the Nightosphere," "Memory of a Memory.")
When asked why he's missing some teeth, Word of God said "he bites trees and rocks and stuff. He's stupid."
Lion-O from the eighties ThunderCats series. A pretty justified case since he aged to physical maturity in a sleep capsule during the long journey to Third Earth. Lion-O is a kid stuck in an adult's body and is fairly naive at first. He eventually becomes a good leader over the course of the series.
Lion-O from the ThunderCats (2011) series. The reboot centers on his coming of age travails. His Cloudcuckoolander beliefs alienate him from his own people, but endear him to the rest of Third Earth. He relies on the Indy Ploy and the fortunate intervention of magic stones for many of his successes.
Rudy Tabootie from ChalkZone, although Snap, his trusty sidekick, is a more literal example.
Coop of Megas XLR fits this to a T. The guy might know how to customize a giant robot, but no one ever said he knew how to pilot it. Or pilot it enough that it didn't level Jersey City more times than your typical Power Rangers episode!
One has to question the logic of these so called "child geniuses" like Dexter or Jimmy Neutron or the Test sisters who resort to mundane efforts of acquiring money such as lemonade sales or selling candy despite the fact that their technology would be worth a fortune, or could make far more valuable products. They probably have the technology to replicate whatever they could want to buy.
Come FusionFall, both Dexter and his rival, Mandark, have started their own technology businesses with now publicly known laboratories.
Ben Tennyson from Ben 10, though this case is heavily justified: after all, he is a 10 year old kid using a highly advanced alien weapon he barely understands himself, and he's teamed up with a ridiculously precocious cousin (who is later revealed to have an innate talent for magic) and a Badass Grandpa; anyone in such a situation and with such allies would look dumb in comparison, unless you are a genius yourself. By the time of Ben 10: Alien Force, he has grown out of his idiocy and is revealed to be much smarter than he seems, while his idiotics moments are more caused by his narcissistic tendencies following him becoming a Living Legend.
T.J. Detweiler from Recess, though his unintelligence comes more out of laziness than actual stupidity. He's a genius when it comes up to coming up with plans, but he's extremelyBook Dumb.
Wally and Gus from Rocket Monkeys. Gus is the more sensible of the two, but that's not saying much.
Arguably deconstructed by The Pulverizer in Teenage Mutant Ninja Turtles (2012), where his idiotic tendencies and desperate desire to become special leads him to not only join the Foot Clan, but deliberately expose himself to Kraang mutagen, despite the efforts of the turtles to stop him. This turns him into the 2012 version of Mutagen Man... aka, an amorphous blob of corrosive goo with his organs floating around inside of him.