Iroh, top to bottom. Looking at the polite old man, you'd never imagine he'd once held the last bastion of resistance to the fire nation in slowly constricting grip. Almost everything he says in adverse situations is well-obfuscated, extremely polite deadpan snark. Also, when he was imprisoned in Book Three, he pretended to be insane while secretly doing exercises in his cell so that, after a while, all of his fat appears to have been replaced with sheer Bad Ass muscle.
In The Legend of Korra, Varrick initially seems to be a goofy, loud, excitable Uncle Pennybags. He then proceeds to escape Unalaq's blockade with incredible ease and an audacious escape plan, and soon thereafter starts manipulating New Team Avatar into serving his War for Fun and Profit schemes while still feigning friendship.
Quinn Morgendorffer (Daria) is a canonical example of the fake-ditz variant. However, it is implied through the series that she hid her intelligence even from herself, for fear of emulating her perpetually unpopular yet brilliant sister Daria.
This applies to all the bullies. Kerney, despite being a teenager and the father of a teenager, apparently is competent enough to pull his weight at church meetings, for instance.
In the episode "Summer of 4 Ft. 2'', Lisa tires of her usual bookworm self and while on summer vacation plays the part of an average anti-intellectual "cool" kid to win friends. She occasionally slips up, however.
Lisa: Ah, a gift from my favourite crustacean! (Gulps) Rick: Hey, did you learn that word from a teacher for something? Lisa: No...I...heard it on Baywatch. Other kids: Oh yeah!...Baywatch!...David Hasselhoff, man.
A very unusual moment of Obfuscating Stupidity comes from the episode "Mother Simpson"; unusual because it comes from the usually dense-as-brick Chief Wiggum. It's revealed by the end of the episode that Wiggum intentionally led the FBI astray in order to help Homer's fugitive mother Mona escape Springfield, leading one to wonder whether the stupid things he did in the episode were genuine or to just throw the feds off track.
In a deleted scene, Homer tells his mother he's trying to bring the nuclear plant down from the inside.
Attentive Simpson watchers and tropers with a good memory alike will tell you that most of the cast of this show has had at least more than a couple "smarter than they let on" scenes, primarily in the series' earlier years and most of its glory days. Although mileage may vary on the definition of putting on an act...a wide range of characters ranging from Bart, Moe, Barney, Chief Wiggum, Homer, to even Ralph Wiggum himself have all had times where their stupidity is treated more like ignorance and/or haggardness than genuine one-dimensional stupidity. Some might even speculate that at one point the series had very few cookie-cutter stupid characters, perhaps as an underlying satire that most human stupidity is rarely involuntary or a natural trait. Homer could have been a man of high class and wealth...but a few mistakes and an unplanned pregnancy later and he sort of "gave up"...making him the oaf we all know today. This aspect of the show however is largely forgotten now, replaced with canned joke-esque stupidity and Homer having a crayon stuck up his nose. Many fans believe the unfortunate loss of this trait to be one of the biggest reasons why the show just doesn't seem as sharp as it once was.
This spills into many other tropes, from the aforementioned Book Dumb, to Genius Ditz, to possibly even one named after Ralph and many others.
Ralph almost singlehandedly took over the town by acting lovably stupid. (we donít actually know the outcome, since the ending was a cliffhanger after the big reveal).
Sargeant Hatred of The Venture Bros. pretends to be a bumbling and friendly villain through season three because he knows being nice to Doctor Venture is the best way to get at the Monarch without violating guild rules.
Season four, however, reveals that Hatred is a genuinely incompetent and fairly stupid man, just like everyone else in the show.
In the Disney cartoon American Dragon Jake Long, there is Spud (an apt name, considering his displayed personality). Spud has the personality of your average idiot (and sometimes worse), but once, when tricked into doing an aptitude test, he gets a perfect score and is sent to a school for geniuses, where he also is far smarter than any of those people too. Spud explains that he does so because he doesn't want the pressure of having to perform to live up to the expectations others might have toward a young genius. He'd rather enjoy life as an easy-going goof.
Baloo from TaleSpin (and to some extent, the original The Jungle Book as well) often seemed very dense and unsophisticated, suffering from endless financial problems and even having trouble spelling correctly. However it was shown many, many times that Baloo is probably a certifiable genius, and it's not just his flying skill (which would put him in line with the Genius Ditz) although that alone is certainly something to boast. Baloo shows an uncanny knack for geography even when navigator Kit isn't around, seeing through deception, insight, philosophy, and even mechanical skill and an abstract understanding of machines (though not to Wildcat's extent obviously). All the more impressive when one realizes that he does all of this based off of instinct, as his denseness is mostly attributed to being largely uneducated (which was somewhat more common in the 1930s when the series takes place anyway), as well as his own lazy nature.
Sarah Palin, in South Park's take on the 2008 US election, proves to be far more intelligent when outside the public eye.
Ike of all people if you skipped a few seasons. It is most obvious in "About Last Night...", but by "Fatbeard" it has either become canon or Only Sane Man territory with nobody noticing.
The Penguins of Madagascar had a walrus named Rhonda, who started to live in Marlene the otter's habitat as her "roommate". Rhonda proves to be quite an annoyance to Marlene eventually, and she demands the penguins to get rid of her. However, when the penguins did get rid of Rhonda, it turns out that they were right about her being a spy for Dr. Blowhole.
Private's uncle Nigel used this as a cover to hide the fact he was really a super spy. At least until he was caught by The Red Squirrel, who refused to believe it.
In an early episode of Kim Possible, Kim rescues Nakasume, a Japanese toy manufacturer, from Drakken and Shego. Nakasume conveys his thanks to Kim by whispering into his secretary Yoshiko's ear, who then translates. Nakasume appears later in the Kim Possible movie So The Drama, in which it is learned that he is actually fluent in English, but enjoys whispering in Yoshiko's ear.
In the Season Four episode "Larry's Birthday", Professor Dementor's henchmen kidnap Kim's RPG-obsessed cousin Larry (wrongly thinking that he was Ron) as part of Dementor's plan to steal Kim's battle-suit. Larry acted like this was just an elaborate RPG scenario and went along with the kidnapping, seemingly siding with Dementor when he managed to kidnap Kim and Ron. At which point Ron (who has played against Larry before) smiles and asks, "Yo, Lar! We looking at a Scenario Nineteen here?" Larry answers, "Precisely," and reveals that he was wise to Dementor's plan and had been wearing Kim's battlesuit all along to keep Dementor from stealing it, and even uses the suit to take Dementor down.
In Justice League Unlimited, the Question's conspiracy nut persona is probably an act. At the very least, he isn't entirely imagining things, or having delusions of grandeur about his ability to gather info - this is highlighted by the fact that Batman asks him to ferret out a link between Cadmus and Luthor. When he attempts to assassinate Luthor to stop Superman from doing so, he notes that the Justice League's reputation will survive this otherwise unconscionable action because he is, in his words, "a well-known crackpot".
He then proceeds to spout conspiracy theories in response to every question he's asked under torture, in a more amusing equivalent to a soldier's 'Name, Rank, and Serial Number.'
The Flash did it first, though it wasn't really expanded upon until the Unlimited episode Flash and Substance, where he demonstrated that his happy-go-lucky persona hid a hero who was, perhaps, even more in control of his home city than Batman, all the more impressive because he achieves it all with a smile.
It is implied in Ben 10: Ultimate Alien that the titular character was doing this to some extent throughout the Ben10 original series in order to cope with suddenly having to deal with interstellar and supernatural threats on a daily basis as a ten year old boy.
This was Bugs Bunny's strategy in Barbary Coast Bunny. After Nasty Canasta stole his gold, Bugs the Karmic Trickster pretends to be a hopelessly naive hayseed during his visit to Canasta's casino. He promptly bankrupts it by effortlessly winning big at every game of chance.
In The Grim Adventures of Billy & Mandy, a new kid named Bobby takes over Billy's position as the stupid kid because everyone was sick of Billy, as it turns out Bobby is only pretending to be stupid in order to replace him at his house hold.
It's not much of this case (considering he really is stupid), but when SpongeBob SquarePants' friend Patrick aimlessly walks straight into the lagoon while holding his rare trading card, SpongeBob asks him why he did so. Here is Patrick's answer:
Patrick: SpongeBob, you can't always expect my usual brand of stupidity. I like to mix it up. Keep you on your toes.
An earlier example is in I'm With Stupid, when Patrick wants to impress his parents, SpongeBob fakes stupidity.
The Darker and EdgierG.I. Joe: Resolute premiere has Cobra Commander stating he was pulling this trope in earlier series, but it didn't work so he's dropping the disguise and getting serious.
It's notable that Cobra Commander kills a lot more people and is much less tolerant of his subordinates attempting to pull crap on him, but otherwise acts just as bonkers as his previous 1980s incarnation, which at least suggests the possibility that he's just saying that to save face.
He's certainly Genre Savvy enough to have backups for both the super weapon and communication jammer, and a bomb shelter that can resist a blast from said super weapon. He is still surrounded by incompetents who went to the Storm Trooper Academy for Accuracy or mouthy psychotics like Zartan and Storm Shadow. They only failed because GI Joe were slightly more competent and accurate. It's quite a different kind of bonkers though; in the old cartoons he'd have some idiotic plot to rule the world by rigging a mayoral election or tricking some rich old man into marrying The Baroness using mind control perfume. In Resolute? He destroys Moscow without warning and hacks his own troops apart.
Although this is crossing into comic book territory, it should be noted that Cobra Commander's original comic-book persona...predating even the original cartoon...was ruthless and crazy since the beginning. From secretly brainwashing his own high command in case they turned on him, to managing a Cobra that actually manages to kill more than a couple of established GI Joes, to even killing his own son Billy in later comics...it's easy to see why the Sunbow cartoon had to tone him down into the bumbling fool we all knew and loved as kids. You don't mess around with Cobra Commander in the comics! As Resolute attempted to go back to his original persona more, the speech about pretending to be a bumbler could be considered something of a nod to this extreme difference.
Bobby's World played with this from time to time. While Bobby was, for the most part, genuinely naive, there were times when he would overplay it to cause problems for Derek or Kelly, while appearing innocent. One notable example was when he repeated everything Kelly had said about her ex-boyfriend (who she still liked), in front of said ex-boyfriend (who had come to apologize after their fight), while pretending to not know what it meant (needless to say, he ceased to be apologetic).
Steve's friend Barry from American Dad! he is generally seen as a very unintelligent individual. But in some episodes he is shown to be very good with mechanics; when asked about this, he replies that he doesn't know how he did it. Another episode reveals that he isn't as stupid as he seems: he is actually a very intelligent psychopath who takes pills to control this side of him.
It's very difficult to tell, so sometimes one also wonders whether Francine shows this; she's often shown giving basic answers to complicated issues and spouting non sequiturs at the most inappropriate moments, but she's also shown signs of brilliance; she's a natural at pretty much any form of business, and has almost always been right in arguments against Stan, although that isn't really saying much. She did, however, lead an entire handicapped mafia to their deaths.
Total Drama World Tour: Sierra is smarter than she lets on. As part of her being a extremely devoted Loony Fan, she knows the show's rules back and forward, and is extremely Genre Savvy; the most stand-out form of this is that she pretends to become Heather's latest pawn while secretly planning to turn it on her.
An episode of Garfield and Friends has Garfield pretending to be an idiot in order to get back some money another cat stole from Odie, and Garfield uses play money to bribe the other cat into giving the money back.
Looney Tunes: Slowpoke Rodriguez is barely mobile and doesn't think about much aside from his next meal. However, he quickly proves to every bit as dangerous as Speedy Gonzales, thanks to being a crack shot and an expert at hypnotism.
Young Justice: Every member of the Flash family (with the possible exception of Jay). Yes, they're a bunch of happy-go-lucky jokesters, but they are also extremely intelligent when it comes to science. Which is to be expected, given that they got their powers from a science experiment. This is taken Up to Eleven with Impulse, who acts like a little kid in a candy store for his entire introductory episode, after which The Stinger reveals that it's all an act he uses to hide the fact that he comes from a Bad Future.
An episode of Pepper Ann culminates in several characters revealing secrets about themselves including Cissy Rooney, who admits that she plays up the Dumb Blonde stereotype because she doesn't want people to be intimidated by her intelligence.
Harleen Quinzel may seem to be just another dumb blonde, but before she embarked on a life of crime, she was a psychiatrist at Arkham Asylum. The few times she's not speaking "in character," she speaks with what is arguably a refined English accent, suggesting that she came from "good breeding." Among other things she's done that suggest that she's smarter than your average bear, she has gone toe to toe with Mercy, Lex Luthor's highly trained bodyguard/personal assassin, in hand-to-hand combat on repeated occasions, eventually coming out ahead (well, Quinn was able to walk away, even in handcuffs, Mercy wound up in a hospital bed). She also, upon seeing that Joker was really depressed on one occasion, came up with a plan to capture Batman, and then go that extra mile and actually successfully execute it.
As stupid as Dum-Dum is on The Perils of Penelope Pitstop, he came up big on Penelope's behalf twice. He used a feather to rescue her from a man-eating plant (throwing it into the plant so it caused it to gag and spit Penelope out) and the nabbing the Hooded Claw's hat and cape so Penelope could win a scavenger hunt. Penelope kissed Dum-Dum each time for his efforts.
On The Weekenders, when the gang were looking for dates for the school dance, Lor's suggestion for attracting boys was to pretend to be stupid.
Fitz, part of the Alliance's Detail Team, frequently does this to further the coverup.
Sid, a crazy Conspiracy Theorist in Roswell, is actually a former Alliance operative, and a Shadoen infiltrator.
Cosmo from The Fairly OddParents is hinted at to be this. Multiple times, in fact. Cosmo was actually the more intelligent one at the very beginning of the series run, he sometimes has random bouts of intelligence seemingly from nowhere. Only to immediately follow it with another stupid moment.
Stan Pines of Gravity Falls. Through all of season one, he's presented as totally clueless to the various supernatural occurrences around him, even though he personally experienced some of them. Then the season 1 finale comes around, and we find out he's the owner of Journal #1 and now 2 and 3, implying it was all an act. Confirmed in the Season 2 premiere, where he admits it was a deliberate act.
Perry the Platypus in Phineas and Ferb does this. He pretends to be a domesticated animal, but every day he goes to work as a secret agent thwarting the schemes of a Mad Scientist.
Every so often, Wander of Wander over Yonder lets his mask of being an oblivious, daydreaming Man Child slip off and reveal that he knows exactly what's going on around him and how dangerous Lord Hater is.
An episode of Family Guy depicts Lauren Conrad as a closeted genius who, among other things, created a real-life Genesis project. She wants to keep her intelligence a secret because "America doesn't like smart people," which is why George W. Bush got elected twice.
Motorcity: The Duke of Detroit's soldiers do this in the finale, where they cower and generally argue with each other to be much of a help when the heroes (the Burners) are being attacked by Kane's forces, one even notes that his laser has been jammed up with gum. It's later revealed that this was all part of a complicated ruse to get the heroes to think the Duke was weak, and when he actually teams up with the Burners, the guards go on the offensive and successfully fight back the Kane robots and troops.