David 'Walky' Walkerton in It's Walky!; while part of his scatterbrained playfulness was due to brain damage and psychological manipulation, it was also at least partly intentional; at one point, Joyce points out that he had been an honors student in high school, which he immediately denies.
Depending on how one interprets his general tactics and his speech in the Order of the Stick prequel book Start of Darkness to Redcloak after tricking the goblin into killing his own brother, Xykon also applies.
Xykon is more Competent But Lazy, he goofs off a lot of the time due to his arrogance, and incredible power level. He just takes advantage of people underestimating him when it occurs, often to do a little Hannibal Lecture here and there
When he first turns Xykon into a lich, Redcloak uses his holy symbol as Xykon's phylactery. He plans to use it to control him, and reasons that Xykon is too stupid to realize it. Then, when he threatens to destroy it, Xykon reveals that he knew about his plan all along, and that destroying his phylactery won't harm him unless his body is destroyed.
Right-Eye (Redcloak's brother) has a daring plan to kill Xykon for what he's doing to the goblins under his command. Redcloak kills him rather than let him destroy Xykon, since he needs Xykon for his plan to work. Then it turns out that Xykon knew about Right-Eye's plot, had a magic ring to protect himself from it, and didn't stop him before because he wanted to see if Redcloak was loyal enough to do it for him.
Shojo. The man spent years pretending to be senile to trick his nobles into believing themselves to be competing Men Behind The Man so he could rule himself without fear of assassination.
Invoked by Tarquin, who argues that his son's greatest weakness is an unwillingness to let anyone think that he is a fool.
Liquid Snake of The Last Days of Foxhound was initially a somewhat arguable case. It had been implied that despite his borderline idiotic behavior he was in reality a killing machine who has only been reduced to his current idiotic state because he suffered brain damage just before the start of the comic. Later it became less arguable as Liquid recovered somewhat and continued playing the dupe until he could attempt a revolt. Even towards the end of the comic he still falls short of his original portrayal in Metal Gear Solid, where he pulled this off quite well.
Helene from Avalon is a Clingy Jealous Girl, seemingly oblivious to the fact that her boyfriend, Alan, just wants to get rid of her (not to mention his interest in Ceilidh). In one of the last drawn strips, she tells Joe that she's known Alan's feelings the whole time, but the fact that he hasn't told her makes her think that he cares about her enough to be won over, and after being forced to admit this, gives up on him.
The interpretation of madness given in Narbonic explores this trope at length.
And this makes one wonder about Red Mage, and thesethreemake one wonder about Fighter. Of course, actually being as dumb as Fighter would be a far greater feat than pretending to be that dumb.
The second possibly shows a variant of this - he's not as dumb as he appears to be, he just never feels the need to actually think about anything beyond his whimsical Cloud Cuckoo Lander ideas, one of which, remember, worked like a charm ("Sword-chucks, yo!"). A fighter actually using his brain is a threat to be feared.
The possibility of Fighter pretending to be dumb is lampshaded in this strip.
Red Mage is well...a Red Mage (third highest Int stat in the first Final Fantasy game), but he is too much an eccentric, stat obsessed, narcissist to concentrate on doing anything remotely competent for a prolonged period of time.
Master Payne: You cannot possibly be as stupid as you act.
Oggie: ...Ken if I vants to be!
Another example can be seenhere. Announcing your presence with a loud fanfare and a Slow, Badass March is of questionable intelligence in a world were massive death rays abound. Using said march to distract the enemy soldiers from the fact that most of the horde is already in the city and wreaking havoc...
The best thing is, in those two strips, she first confesses to using Obfuscating Stupidity, then she fools the man she confessed it to.
It gets better. She outsmarted Lucrezia. No, really. Zola is the Queen of this trope.
Well, it's more accurate to say she's one of the few examples of multilayer obfuscating stupidity. Initially the stupidity she was feigning was pretty obvious in it's fakeness, but then she dropped into a much more natural seeming smart but rather gullible act and it was only when she went up against Lucrezia that she finally dropped all the acts and revealed just how much cunning and treachery she had been hiding.
Baron Wulfenbach, however unexpected. He's clearly in fever. The moment a fresh minstrel from the street is in his room, Klaus knocks it off and express-interrogates the guy.
Anak from Tower of God, who occupied the throne of the Crown Game acting like a selfish little princess, but chose that place because unknown to even her team mates, she was in a tactically superior position.
This strip of Darths & Droids reveals that Jim, the rather dim-witted player doing Qui-Gon Jinn, is in fact studying geophysics. He acts as stupid as he does roleplaying because he uses it as downtime and turns off his brain.
And now that Jim is playing Han, he's deliberately playing him even dumber as a character trait ... and possibly for subtler reasons of his own.
Penny of Penny and Aggie, as the comic's Lovable Alpha Bitch, often acts ditzy in front of others. (She tells her friends, for example, that math is "fundamental" because one day you might find yourself stranded on an island "with a bunch of numbers that don't mean anything.") However, she's one of the school's top scorers on the practice SAT.
In the "Tumbledowns" story in Tales of the Questor, Guardsman Mulharney seems a hardass constantly on Quentyn's case while he's going undercover to recover his stolen sword to the point where the young Questor was forced to physically attack him to maintain his quest. However at the end of the story, Mulharney admitted that he knew who Quentyn and correctly guessed he was doing something important from the start. Therefore, he played dumb to give the kid street cred to accomplish whatever he was attempting against the street gangs he was infiltrating.
Buck Godot has a rather shocking case of this in the person of the Winslow, as revealed at the end of the Gallimaufry storyline.
Looking for Group has Richard. He may seem like the insane character foil of Cale, but it's been revealed that he's really much smarter (and maybe less insane) as we've been led to believe. I mean, he figured out that his rabbit was the Archmage, something nobody could have guessed and he's apparently been following Cale since the very beginning, not because of boredom, but because he knows the guy is going to become something big..
For example, he is the mayor of a small village, which got under siege at one point. He askes his second hand if they "launched all the women and children at them." with catapults, to which he replies that they did but it didn't have any effect. That plan seems typical Richard, nonsensical violence and all and disregard for human life. But if you find out that the entirety of the village is undead, the plan makes a whole lot more sense and actually works out in the end.
Guineas from El Goonish Shive, a bit shy and lazy guy whose boss gave him only simple work because he avoided assuming his human form and acted as barely more than a dumb rodent. He isn't stupid.
Grace can also appear this way occasionally. She's actually pretty bright, but she's genuinely naive about some things and sometimes pretends to be about others. The difficulty is telling which.
This trope appears to be not only Blade Bunny's M.O., but her whole way of life. The first time the mask slips she glues it back in place by seeming unable to count to four.
Although, thus far, it's not yet clear to what degree it's this trope, and to what degree she's a genuine Genius Ditz or Bunny-Ears Lawyer with a specialty in being a ninja — she drops the "mask" so rarely, even when it's counter-productive, that it could be either or some of both.
As with the Waiting For God example, this seems to be something of a deconstruction since she has been using a display of stupidity as a defense for so long that, though intelligent, using her brains is now always the second option.
Yuffie admits to doing this in Ansem Retort, pretending her English skills aren't as good as they are because it makes it easier to rob people. And fools people into thinking she knows kung fu.
Sgt. Schlock of Schlock Mercenary behaves in a childish, innocent manner and doesn't seem to deeply understand anything...but is actually far more canny and observant than this suggests. This may not be an example, though, as it's unclear to what degree, if any, he intentionally cultivates this rather than it just being how he is.
Gabe from Penny Arcade. Despite his antics and general naivete, he lets on that he may be smarter than most people think. He just spent years of his life faking stupidity so that his parents would stop calling and asking how to fix their computer.
Grymm from Voodoo Walrus seems to be playing at this since he always seems to be lost in a haze of obliviousness, and yet he can still step up and solve problems when least expected and pull part time duty as a self-centered, greedy, manipulative superhero.
Except that recently there's been evidence to suggest that Grymm and the masked T-Square couldn't possibly be the same person.
The Trenches: After being turned away by multiple potential employers for being "overqualified", Isaac resorts to this.
Isaac: Use small words, Isaac. Use small words...
The governor of Beldatz in Nahast: Lands of Strife. Considering that there is a legendary organization of spies watching his every move - directed at least in part by his wife, no less - it's quite impressive that he manages to maintain the deception. Additionally, his daughter is part of the organization watching him.
In Homestuck, Jake pretended to be unaware of his friends' feelings in order to avoid having to give an answer before he knew how to respond, which fit well into his general sort of clueless personality. However, when he believed Jane telling him she didn't have feelings for him, he was only fooling himself.
In Magick Chicks, there is doubt in-universe and out about how much of Jacqui Brightmoon's ditziness is real (with a bit of genius) and how much is an act.
In Frederick The Great this is Ulysses S. Grant's whole shtick. He pretends to be an idiot lazy drunkard to distinguish himself from the slicker generals in the East and thus when they inevitable screw up it will be his rougher attitude will get him redeployed.