Calvin is apparently a user of this trope, as he once told Susie that it's far easier to keep people's expectations low, and wow them every now and again, then to keep them high and wind up disappointing at some point.
A cloning storyline in which Calvin (supposedly) creates a personification of his "good half" to take his classes for him proves this: if the clone is real, than it demonstrates that Calvin could do well in school if he bothered to try; if it's an extended game of make-believe than Calvin really is doing well for a change (if only for the sake of keeping the game going.) Then again, any of Calvin's musings to Hobbes on the nature of existence and reality during any given 'sledding' strip should tip even the most casual reader off that the kid's a freaking genius, it's just that school bores him senseless.
Some incarnations of Superman (especially the iconic portrayal by Christopher Reeve) had their Clark Kenting rely almost completely on Clark Kent being a clumsy, timid stick-in-the-mud (albeit not stupid), so nobody would seriously entertain the notion that this farm boy could be the Man of Steel. Indeed, it was a running joke throughout the Golden and Silver Ages that Clark couldn't get a date with Lois Lane because she was only interested in the brave, physically-capable Superman.
Superman is the Trope Codifier, for DC comics. The fact that he can disguise himself with a pair of glasses and this, and nobody ever figures it out themselves, is a testament either to human stupidity or his awesome Obfuscating Stupidity. Oh, and the hair curl. This is even lampshaded in Lois and Clark: the New Adventures of Superman, where Lois is told point blank, "Hello! Duh! Clark Kent is Superman!"
In one Silver Age story Lois herself used this trope; while disguised as a blonde to get close to a story she bumps into crooks who notice her 'resemblance' to Lois Lane. She pretends to be a Dumb Blonde gangsters' moll and goes along with the crooks plan to use her as a 'fake' Lois to trap Superman.
In All-Star Superman, it's pointed out that Clark exploits the identifying power of mannerisms and posture to better his disguise. Luther even outright says that if Kent stood up straight and worked out more, he could have a body that looked like Superman.
An interesting subversion came when Lex Luthor fed all the evidence he had regarding Superman's identity into a supercomputer, and it told him that Superman's secret identity was Clark Kent. Luthor then destroyed the machine and ignored the results, but rather than thinking that someone as bumbling and timid Kent could never be Superman, he believed that someone as powerful as Superman would never demean himself with such a disguise.
This backfires on him during No Man's Land when he tries to secure government aid for Gotham as Bruce Wayne. This fails for various reasons: 1) Lex Luthor is pulling strings to keep Gotham from getting help in a bid to buy up its real estate, 2) the problems the earthquake brought to Gotham can't be solved with money alone, and 3) no one takes the "idiot playboy" Bruce Wayne seriously. It really doesn't help that it looks like Bruce ditched Gotham and moved to the Bahamas using his money and connections despite the lockdown. In the end Bruce has to rely on Lucius Fox to secure government aid while he as Batman deals with the problems in Gotham and Lex Luthor's machinations.
It becomes a major plot point of 'Batman: R.I.P.', which starts with Alfred pointing out that Bruce is using his 'Batman' voice even when he's out of costume in the Batcave. And after a period of dating Jezebel Jet, she confronts him during a dinner with the question of whether or not there's anything more to Bruce Wayne than the fun, spontaneous, but ultimately shallow man she'd been dating. The relationship ends up being saved by an assassination attempt on Jet, with the assassin even writing off Bruce as a layabout "...with Bowflex muscles." But Bruce manages to piss him off enough to get dragged off into a secluded room by a henchman to be 'executed,' at which point he sticks to the shadows and picks off the assassins without them ever seeing him. Unfortunately, when he's interrogating the leader, Jet sees him illuminated by moonlight and, between the physical evidence and her own belief that it was impossible for there to be so little to a person, she realizes that he's Batman, in spite of Bruce's hate-filled tirade against himself for the exact sort of shallowness she had perceived. Of course, she already knew his identity, and this was all a Evil Plan by the Black Glove.
The Penguin. So good it even fools some writers. Penguin was the first villain to actually escape from Batman and outsmart him, and it was a running theme in all of Penguin's first stories that he always somehow managed find a way to escape. Truth is, people too often forget that the Penguin's shtick is that you're SUPPOSED to underestimate him. You're SUPPOSED to be so underwhelmed by his ridiculous mannerisms that you never see the scheme coming before you've fallen victim to it. There is a reason the umbrella was chosen as his weapon. It serves as a metaphor for the Penguin's character and nature. Like his umbrellas, the Penguin appears completely harmless perhaps even mundane, but also like his umbrellas he conceals a darker nature.
Quote from Bruce/Batman to Tim Drake in 'Penguin Triumphant': "Everyone seems to consistently underestimate the Penguin — myself included. In point of fact, Cobblepot is ruthless, vindictive, calculating, inventive — and perhaps the most brilliant man I've ever fought."
It's also true of Spider-Man. In the early days, no one would have suspected bookish, shy Peter Parker of being the web-slinging, wise-cracking Spider-Man. This remained true as he ended up a science teacher.
This is one of the few cases where the "stupid" act is part of the actual superhero persona more often than the civilian/secret identity. Peter Parker is well-known as a bright if not brilliant and educated man who avoids physical work whenever possible, where Spider-man is a dumb jock who solves problems by punching them in the face and taunting them, kind of like a kid. All of the technical and scientific aspects of the hero-persona's work are only revealed to other heroes if then. An example would be explaining a complex network of bugs, tracers, and detectors used to track targets intentionally allowed to escape a fight in a marked vehicle as "spider sense" and getting lucky.
Grunge of Gen 13, both pre and post-Worldstorm. The post-Worldstorm versions works very hard to give the impression that he's a stupid slacker, due to bad experiences when he was younger and still extremely nerdy. Pre-worldstorm, he takes the same advanced biology classes as the team's resident genius, Fairchild.
Grimlock of The Transformers (at least in the Marvel comics) was depicted as affecting his speech impediment to both make opponents underestimate him and because of his own belief that intellectuals are inferior.
While it's not always clear that he's doing it on purpose, it's been said more than once that one of Deadpool's "superpowers" is actually the ability to distract and confuse people by rambling on and on about stupid things until you either want to surrender or commit suicide from all the inanity - and yet, the whole time he's rambling, he's efficiently killing or otherwise getting on with whatever task is at hand. Often people underestimate him as a complete idiot because of all the talking, although occasionally someone will say he's an idiot but not underestimate his fighting abilities.
For example, his irritating, non-sequitur monologuing and erratic behaviour is exactly why he could beat the crap out of The Taskmaster. Not only because Taskmaster became so irritated by the constant flood of inanity that he got all distracted, but also because he couldn't copy Deadpool's unpredictability and insanity. And Deadpool knew precisely what he was doing by unleashing it in spades. But yeah, mostly he's just nuts.
During Civil War, he used the phrase "addled moron that I am (or pretend to be)". This being Deadpool, it's entirely possible the "or" should be taken at face value, and he's genuinely too crazy to know how intelligent he is at any given moment.
Deadpool also says to Cable in the Cable and Deadpool comic something along the lines of " It's fun playing the fool, but do you know when it's stops being fun? When you actually start to believe it."
This is deliberately shown in one strip, when Odie watches Jon, then Garfield exit the house with a wicked smile...only to settle down in a smoking-jacket, pipe, in a plush recliner, watching a show on Classical music with a copy of War & Peace nearby.
And in another strip where John, Garfield and Odie go on a picnic and Odie "accidentally" locks the doors of the car, "trapping" himself inside while John and Garfield try to instruct him on unlocking the doors. The final panel has it pouring rain, Odie enjoying the picnic meal while listening to the radio, and John and Garfield stuck in the rain wondering if Odie's not as dumb as he appears.
Another strip has Jon struggling to solve a Sudoku puzzle and giving up, Odie examines it and solves it very quickly to Jon's surprise.
11-year-old Molly Hayes, the youngest of the Runaways, seems to have the mentality of a six-year-old most of the time. However, she sometimes reveals herself to be at least as mature as her teammates, who are all in their mid-to-late teens. At one point, Molly's telepathic father states that she "acts childlike to lower people's defenses", but actually has "a ferocious intellect".
Reaches its funniest point when time-displaced Geoff Wilder calls her on it.
Molly: Please, mister! Don't hurt me!
Wilder: Skip the waterworks, kid. Your cloying Rudy Huxtable routine is just an act you put on to get attention from your older friends. Why don't you behave like the bright young woman we both know you are?
Molly: Fine. Your son took after you, you know. He was a total frickin' failure.
Booster Gold acts like a publicity-seeking fool to cover up the fact that he's been tasked with protecting the timestream, since this would bring about Kill Him In His Crib. Although he's not exactly a genius, he is a much more competent superhero than he seems to be.
Booster's longtime buddy Ted Kord/ Blue Beetle II invoked this as well; he was a wisecracking prankster who was also a technical genius with his own company.
Plastic Man is usually portrayed as being genuinely a bit dopey. During "World War III," Grant Morrison's final story arc for JLA, however, he reveals that, thanks to his longtime friendship with a C-List Fodder hero named the Red Bee, he knows just about everything there is to know about "apian management." Since an alien Evil Overlady named the Queen Bee is taking over New York City, and all the big-name heroes are busy on the Moon, Plastic Man ends up masterminding their victory. Big Barda even mentions how out of character this is for him, remarking, "This almost seems like a plan." To which he responds (while disguised as a big clown), "I only act dumb, sister."
Almost identical to another moment in "I Can't Believe It's Not the Justice League" where Dibny tells Booster Gold that "the difference between you and me is that I only act like an idiot".
Near the end of Strangers in Paradise, it is revealed that Casey was doing this all along; while posing as a Dumb Blonde aerobics instructor, she was actually keeping an eye on Katchoo for Tambi. This may or may not be an Ass Pull.
Jack Point, the titular character of Judge Dredd spinoff The Simping Detective, dresses as a combination of a stereotype Private Eye and a clown. His reasons are threefold: To get criminals to underestimate him, conceal gadgets and weapons in his clown gear and to allow bloodstains to be easily washed away. This is also on top of his penchant for whiskey.
The main Dredd strip gave us a particularly tragic version in the form of Tweak, an alien in the "Cursed Earth" story. Tweak was a genius and the ruler of his planet, but intentionally entered slavery on Earth and pretended to be a dumb animal to prevent the evil, greedy humans from returning to his planet in order to strip it of resources and enslave his species.
There's also PJ Maybe, a child prodigy who puts on a big show of being borderline retarded so as to avoid his teachers taking any interest in him. By the age of 14, he's already Mega-City One's most dangerous serial killer.
Averted in the case of the Wally Squad's Dirty Frank. He really is that crazy, but is dangerously effective nonetheless, in part because nobody thinks such a maniac could possibly be an undercover Judge.
Ben Grimm of Fantastic Four often acts like a dumb palooka who doesn't understand Reed's "big words" and loves to clobber. But he's well-educated - a trained test pilot and astronaut (who happens to love clobberin'!)
Ultra Boy, Jo Nah, of the Legion of Super-Heroes, is presented this way in the reboot continuity, partly to explain why he's a dumb jock sometimes, and a clever guy other times. The cover story is that he figured out early on that of the LSH's enemies was manipulating events, and he presented himself as dumbish to keep the enemy's guard down.
To elaborate, Ultra Boy figures out in one annual that Glorith has been manipulating events in the time stream and had used her time powers to usurp the place of the Time Keeper in history, creating the Legion as her patsies to keep other potential universe rulers, like Mordru, weak. Jo Nah manipulates Glorith into a terrible confrontation with the Legion, who sacrifice much but prevail in the end, causing Glorith to lose the bulk of her powers. Because of the time based abilities of the foe in question, Jo Nah has to act stupid for the rest of his life or face repercussions. It doesn't work. Glorith figures it out and displaces Phantom Girl, Jo's girlfriend, in time where she winds up amnesiac and working for L.E.G.I.O.N. Yes people, Geoff Johns is out there, repeating your storyline.
Dream Girl, Nura Nal, acts like a flirtatious Dumb Blonde in public because it makes people underestimate her. She graduated top of her advanced science class (enough to keep up with Brainiac 5 at times), and is a brilliant fighter.
Roger Fox of Foxtrot is usually a quite legitimate Bumbling Dad. However, from time to time he is able to use this apparent cluelessness to get what he wants. For instance, in a week-arc where his wife forces him to go to an aerobics class, he spends the entire time doing embarrassing things like doing the wrong moves and singing along to the music. She gets so mad that she tells him she's never going to take him to another class...which, as his thought bubble points out, is just what he wanted in the first place. In another strip he messes up cleaning the dishes so badly that Andy declares that she will handle all dishwashing in the future. Roger then silently muses that "sometimes having no knowledge is power". Andy then discovers that Roger messed up another chore...
According to one strip of Walter Moers, infants are not only able to talk, but discuss complex topics of philosophy, psychology and the like and only use this trope if adults are around. "Ducky make toot!"
A one-shot Star Wars Expanded Universe comic published in the Tales series depicts a nervous, stuttering freighter pilot landing at a backwater space station for repairs to his ship. After demonstrating his skill in a gunfight, the unfortunate man is coerced into an Old West style showdown with the local crime boss, who is always on the lookout for promising new opponents. The crime boss, Shoto Eyefire, wore body armor and had snipers stationed above them to kill his opponent if he was in trouble. And with these odds stacked horribly against him, the pathetic, stuttering pushover pilot shoots Eyefire through both knees and in the shoulder and fights his way out of the fortress. The Reveal comes when he opens a fake power pack for his blaster to reveal a lightsaber.
Eyefire: Non-lethal wounds...stun grenades...stun beams...lightsabers...I HATE Jedi Knights.
Cinderella acts like a slutty Valley Girl in public, but is really a highly trained Black-Ops agent.
That bumbling little woodcarver who the Emperor inexplicably likes? He's the Emperor's dad.
Done twice in Les Nombrils: First in volume 3, when John John reveals that he does know how to read, and has been deliberately failing classes to prolong high school, then in volume 4, when Karine uses this to make Melanie drop her guard.
The first is Paperinik himself: in his civilian identity of freakin' Donald Duck he acts like a lazy and bad-tempered idiot, but as Paperinik he's a frighteningly competent Cowl with a penchant for humiliating his foes and dropping much bigger enemies with his bare hands (the temper is still there, he just vents in less obvious and more satisfying manners). It's most evident in the second series story "All and Nothing", where Donald at one point has to make a conscious effort to not act as Paperinik while in civilian identity, and some of the non-Paperinik New Adventures Italian stories (the early and the more darker of the modern ones), where he intentionally hams up his apparent incompetence only to outsmart Scrooge (who, by now, reacts to Paperinik showing up with an obvious Oh, Crap, especially if Paperinik has reasons to be pissed at him), Gladstone (who was once led to believe that Donald had successfully took away his outrageous good luck and duped into buying a manual on banishing bad luck he had previously mocked), and everyone else;
the other is Xadhoom, a perpetually furious Physical Goddess with an outrageously high IQ (she comes from Xerba, where everyone holds at least a doctorate and you have to be incredibly smart to get in politics. She ruled it after proving herself the greatest astrophysicist in Xerba's history and topping the previous greatest astrophysicist's greatest achievement at a very young age, before testing on herself a method to solve Xerba's energy crisis and aquiring her powers) who tends to just charge at the Evronians and exterminate them and generally acts as a Blood Knight. Turns out she charges at the Evronians because she knows they can't harm her (and they only try and fight her only when desperate or believing they have something that could defeat her. Even those times they're right, she easily outplays them), has learned a branch of Evronian science that has no equivalent on Xerba, can easily see through Evronian traps (and still charges at them because she knows they'll fail, or, in one occasion, plays along to see when they'll realize she saw through their act), has an amazing self control, and the one time she played a stint as The Chessmaster she fooled everyone who didn't know beforehand. The Evronians realizes early about her origins and real identity (and thus her smarts), but believed her grief and rage had dulled both her intelligence and self-control until too late (their Emperor even admitted they underestimated her big time exactly one panel before losing the only leverage he had on Xadhoom to keep her prisoner).
Captain America, of all people, has done this for about a decade. He, dead serious here, for ever since their existence has... acted like he can't use a smartphone... Yeah. Why, exactly, wasn't explained, but the man is one of the top tacticians on Earth.