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Flaming Carrot from the eponymous Flaming Carrot Comics, actually described in his origin story as "quintessentially retarded". Surprisingly, for some people he's actually a mentor and on occasion it even gives him the "superpower" of Zen Stupidity.
Enemies of the Green Lantern Corps attempted to discredit them by giving counterfeit power rings to several idiots. One of those idiots, a G'newtain named G'nort, uncovered their plot and helped defeat them. He was rewarded with membership in the real Green Lantern Corps.
His Batman: The Brave and the Bold version takes this even farther, as he got his (admittedly unimportant) job in the corps because of his uncle and could only remember The Oath with a cheat sheet.
Specifically, his original concept was that he was an incompetent GL who got into the Corps due to nepotism. Eventually it was retconned to "reveal" that his uncle had actually gotten him into a fake GL Corps (which he thought was real). The retcon was presumably intended to explain how on Earth the Guardians (who never much cared for individual GL's opinions, much less familial relationships) would ever give such an idiot a power ring or let him keep it, but it was still very much played for laughs, including showing several members of the idiot GL Corps.
Lacking ideals other than acquiring mass amounts of cheese dip. Early in the title he inadvertently, and often obliviously, pulled Crouching Moron, Hidden Badass, but now his destructive reputation has grown to the point that armies will flee at the mention of his name, and the writers are running out of foreign lands to put him in to make use of the trope.
Groo is almost always both stupid and unbelievably naive but he has, on occasion, lost the stupidity but retained the naivete, taking him into Skilled, but Na´ve territory (no one disputes Groo's fighting skills, or at least those who do find out pretty quickly they were wrong, often fatally).
Thor's, like his mythological counterpart answer to most every problem is "hit it with his hammer until it goes away" which is largely the reason his nemesis Loki is able to trick him so often. Thor routinely attacks enemies he couldn't possibly beat all the time and only wins due to outside intervention. He once even tried to beat a humanoid alien several thousand times his size this way.
In the Disney Ducks Comic Universe, Donald Duck at times counts (if only because of naivete, acting by impulse, not listening to his nephews or all combined) while his cousin Fethry acts stupidly most of the time.
Al from Marville, who believed that time travel somehow gave him super powers and tries to solve poverty by handing out $100 dollar bills. Unfortunately, his idiocy is not endearing in any way, and more often is just a result of bad writing.