Reggie from Free Birds is a dimwittedegoist who thinks he's better than anyone else. C'mon, the guy doesn't even know what TV is.
The Incredibles: A hero known as Thunderhead was remarked to have been "not the brightest bulb", and even his very name is a play on the term "dunderhead". Indeed, his NSA file in the film's disc 2 DVD gives him the lowest possible intelligence ranking, and it is revealed that he can't read and never finished high school. The Phylange and Universal Man have the same low intelligence rankings.
Po from Kung Fu Panda, while sincere and dorky, is often seen as an embarrassment to his teammates, although he's seen as much less of one after becoming the powerful dragon warrior.
Ironically, he eventually becomes The Strategist for the Master Builders. And not only that, the double-decker couch saves their lives because it was so stupid, the Big Bad missed it. Additionally, the couch is the only object that survives the destruction of the submarine built by the Master Builders. Why? Because, while they're great a building things individually, working together doesn't come naturally to them, and their individual styles conflict with one another.
Also, when Wildstyle is explaining Lord Business' plan, he hears something else, and then he says:
Emmet: I think I got it, but just in case, tell me the whole thing again. I wasn't listening.
Jack Skellington from The Nightmare Before Christmas is probably one of the more justified examples, as his idiotic moments come from not being an idiot per se, but from either not understanding concepts outside of Halloween (the whole Christmas fiasco, thinking the shells were fireworks, etc.) or being so excited for Christmas that he doesn't listen to reason (Sally and her prediction). The only thing he does that is just plain stupid is trusting Lock, Shock, and Barrel to take care of Santa Claus.
In one sense, The Nightmare Before Christmas turns this trope on its head because one of the main causes of all the trouble the characters get into is not Jack's stupidity, but his being smarter than everyone else in Halloweentown. Jack is much too worldly and sophisticated to limit his cultural horizons to his provincial hometown, and his desire for cultural diversity is what motivates him to try to combine Halloween and Christmas. In this light, it's the supposedly naive Sally who comes off as the smartest one.
Santa Claus: The next time you get the urge to take over someone else's holiday, I'd listen to her! She's the only one who makes sense around this insane asylum!
Live-Action
Jake Sully from Avatar. His main qualification for the Avatar program was being his scientist brother's twin. His military training and brash personality cause major clashes with other characters.
Haru: I may not be a great ninja; I may not be one with the universe; but I will say this: NO ONE MESSES WITH MY BROTHER.
Bill & Ted, in every incarnation. While they have an intuitive understanding of San Dimas Time, their basic response to everything is almost always (a) tell a dumb joke, (b) make a rock music reference, (c) going along with what someone smarter suggests, or (d) some combination of the above.
Wikus van de Merwe from District 9. Let's just say that none of his boneheaded decisions throughout the film are out of character. Though it's more a case of Wikus being a Dirty Coward at best and Villain Protagonist at worst.
None of the protagonists in the two Dr. Goldfoot movies are particularly bright, but Franco and Ciccio in Dr. Goldfoot and the Girl Bombs are an Italian comedy duo that make Laurel and Hardy look like intellectuals.
Eddie the Eagle. Eddie's boundless optimism is unquenchable, even in the face of overwhelming evidence that he's completely unsuited to Olympic competition and his pig-headed refusal to accept that might get him killed. He also thinks that he's ready for the next level of competition the instant he has any success at all at a lower level. It's as if he's immune to common sense. It should be noted that this only really applies to the film version of Eddie. The real Eddie was cagey enough to spot a loophole in the Olympic rulebook he could exploit to get to compete, and he was a pretty accomplished downhill skiier.
Forrest Gump. He actually appears to succeed becauseof his idiocy, or rather, because it never seems to occur to him to do anything other than the Right Thing by traditional standards. He becomes a decorated war hero just by trying to save his Army buddy Bubba and his comrades. His shrimping business succeeds only because a hurricane has sunk every boat in the harbor but his; it is strongly implied this is because Gump has been going to church lately and got God on his side. Other characters who do not share his conventional morality, and explore alternative values and lifestyles as so many Americans did in the 1960s and '70s, act like total bastards and/or make fools of themselves. Even the good-hearted Jenny succumbs to death, since she gets AIDS.
Clumsy, slow-witted Bumbling Dad Gang-du from The Host (2006) is a tragicomic example. He's constantly viewed as the family screw-up, and because he's such an idiot he endangers lives without meaning to. He even got his father killed after he miscounted the number of bullets in his gun. Still, he can be a Crouching Moron, Hidden Badass when the situation calls for it.
Harmony: It's bad. Badly is an adverb, So to say you feel badly would be saying that the mechanism which allows you to feel is broken.
(which later leads to)
Perry: Sleep badly. Any questions hesitate to call.
Harry: It's bad. Sleep bad. Otherwise it makes it seem like the mechanism which allows you to sleep is...
Perry: What fuckhead? Badly is an adverb. Who taught you grammar. Get out, vanish.
Lt. Frank Drebin from The Naked Gun is fairly competent, but given he lives in a slapstick world, he's always prone to dumb sayings, decisions, pratfalls (though once it helped, as Frank tripped on the power cord of a bomb and stopped it), and humiliating things.
Vincent Vega from Pulp Fiction is this at best; his idiocy gets himself and a witness to one of his hits killed and also comes dangerously close to getting Mia Wallace killed.
Adam Sandler in most of his movie roles. Billy Madison even has the justification of having dropped out of grad school. Pixels has Sandler excel due to knowing old-school video games.
Ship of Monsters has Lauriano, who doesn't even realize what's going on until the last minute. He wins mostly through luck.
Star Trek (2009) Kirk seems to be playing with this one. Unlike TOS Kirk, the reboot Kirk needs his supporting cast to keep from getting his ass handed to him every five minutes (Spock, Spock Prime, Sulu, Scotty, Bones, and Chekov keep Kirk from killing himself in various interesting ways while Hypercompetent Sidekick Uhura holds the ship together!). Likely justified, due to being raised without a father. On the other hand, when he actually does think he's brilliant. For instance, he immediately figures out that that the Enterprise is heading into a trap when they travel to Vulcan.
Patchi from Walking with Dinosaurs: The Movie was this at first, a chronic complainer with poor survival skills. He finally shapes up when he leads the herd off the frozen lake, the same lake Scowler is leading everyone across.
