Berry from the Foster's Home for Imaginary Friends episode "Berry Scary". She at first appeared overly cutesy and kind, especially around Bloo, who she had fallen in love with, but when faced with obstacles to her perceived "relationship" with Bloo, she dropped the cutesy persona and went off on bouts of jealous rage. Her mask eventually cracked at the end of the episode, when Bloo finally spelled it out for her:
Bloo: Whoa whoa whoa, who said anything about love, Heather? Berry:MY NAME IS BERRY!!!
Rarity sometimes has shades of this as well, her desire to always appear elegant and graceful sometimes leads her to repress her anger and frustration. "Sisterhooves Social" is probably the clearest example, as she spends the bulk of the episode trying to keep a happy face as her sister causes increasing amounts of trouble for her.
The brainwashed Joo Dees (shares initials with Jane Doe). Mentally, they're practically Stepford Wives. Of the government.
Also Ty Lee. At least some of her cheerful and perky attitude is a ruse to keep Azula happy, at least before she betrays her.
Hama. She first appears as a kind but slightly eerie and mysterious old woman. Later we find out that she's a Waterbender, and that she's only living in the Fire Nation because she was captured from the South Pole, imprisoned with other Waterbenders, and prevented from bending her native element for years. She escaped by bending a guard's body fluids, turning him into a People Puppet, and forcing him to unlock her cell. This alone would have been justifiable and wouldn't have made her into a villain. But then we find out that she's been living in the Fire Nation for years just to exact revenge on innocent civilians, capturing them with the same technique she used on the guards. Then, when the Gaang tried to stop her, she started People Puppetteering Aang and Sokka and threatening to make them kill each other. Katara (who had been unwillingly learning the art of "Bloodbending" from Hama) was forced to Bloodbend the old woman to save her friends and Hama's town.
Bloberta Puppington, Orel's mother from Moral Orel, exemplifies the psychotic version of this trope. "Ah, yarn... yarn... yarn ! YARN ! YAAARRRNNN— ... Welcome home, dear !"
Same with Clay, Orel's dad. He sometimes puts on a happy face, but most of the time he's drowning in misery and drunkenness in his study.
Hell, everyone in Moralton, who as a whole are more concerned with the appearance of faith then actually following it. A few get better. Most, do not. The fact that this is based on the creator's childhood experiences is depressing.
Marge Simpson of The Simpsons has repeatedly been portrayed this way, commenting on bottling up her feelings and staying with Homer "no matter what" after he does something truly horrible. Lisa has pointed this out on at least one occasion. This has become more and more relevant as the show has descended into unrepentant farce, to the point that one episode even has Marge admitting that she only stays with Homer for appearances sake, due to the fact that everyone else in Springfield is single, divorced, or in a marriage hit by marital infidelity and as such, the entire town consider Homer and Marge's marriage to be a roaring success in comparison.
One very early episode shows Marge was taught to be one of these by her mother as she tries to make Lisa the same way in hope that if they pretend to be happy, they'll be happy after a while. However, when Marge sees Lisa actually starting to step into the role and how others start to take advantage of it, she gets angry, whisks her away and takes back her advice. Perhaps it could be said that in the series in general Marge has a tiny bit too much will left to stay in this role consistently.
It's worth noting that Marge being in this trope is a direct result of the writers amping up Homer's Jerk Ass qualities. You could accuse Marge of being a Stepford Smiler in regards to Homer even in the show's early days, but a number of episodes have her well-aware of his faults and calling him on it when appropriate. One episode has her say that she sees something in Homer a lot of others don't. Essentially, she looks past his anger, dimness, and other faults to see a generally good guy, which is how he was portrayed back then. However, when the writers started making Homer increasingly stupid and constantly a jerk (as opposed to in fits of anger or simply not knowing better)... well, Marge came to embody this trope.
Lindsey Naegel (aka the businesswoman) appears to be, on the surface, a powerful and successful woman who's sophisticated demeanor hides a dark side (she's a self-admitted sexual predator) and is a functional alcohallic.
Hey Arnold!: Helga's older sister Olga is a Type A version of this. She looks perfect on first glance, being very good at academics, music, has a personality many consider pleasant, and is considered very beautiful. However, with enough pressure this facade of perfection can crack revealing a young woman that's dangerously neurotic and melodramatic due to having to live up to her parents' constant attention and enormously high standards. In Olga's own words: "You're [Helga] lucky they [their parents] don't even notice you."
Later on Home Movies, the rather joyful character Melissa was revealed to be type A, longing for her absent mother to come home and imagines a storybook style reuniting with her despite that her mother may not even wanted her in the first place.
A Slappy Squirrel short on Animaniacs had one in the form of neighborly Candy Chipmunk, who hid her neuroses and obsession with neatness and perfection with a perky facade. Needless to say, Slappy's mental torment quickly strips her of her smile and reduces her to a nervous wreck- all over an argument over recycling. Slappy dropped a can in Candy's curbside bin.
Eve from Alpha and Omega is a type C version of this trope. She is basically a Mama Bear with a creepy smile who is practically caring to her family to the point that she would make death threats towards anyone who would hurt her pups in any way. Even though it comes off as scary to the other wolves including her own family, it comes off as becoming completely hilarious otherwise.
Aunt Bogunda from Mr Bogus appears to be a Type C of this trope, as underneath her smiling exterior, she actually bears the ability to fight back against those who confront her, as demonstrated in the second act of the episode "Totally Bogus Video".
Leela from Futurama describes her method of coping with her tragic life by apologizing for a brief grief-stricken outburst and explaining that "usually I keep my sadness pent up inside where it can fester quietly as a mental illness."
Dexter Douglass' mother from Freakazoid! appears to be a Type B. The writers credited this to Tress Mac Nille's delivery, noting how she could take any of her lines and still sound unusually chipper regardless of what was being said.
Genie from Disney's Aladdin seems to at least somewhat fit this Trope. He mostly acts very upbeat and cheerful, always cracking jokes, and having lots of fun with magic. But at one point we learn he is actually quite sad and really wishes to be free.
Eddy from Ed, Edd n Eddy. He may seem confident and manly (to the point that he refuses to cry or show moments of vulnerability around his friends) but it's eventually revealed in the movie that it was all a mask to hide the inferiority complex he got as a result of years of physical abuse at the hands of his older brother. A mask he made in naive belief that he would be popular if he acted like his older brother who the other kids initially thought of as cool. Even when the other kids see him being abused by his brother Eddy still smiled, rather than cry or scream for help, in a desperate attempt to keep up the charade. Later, he breaks down in front of everyone and they end up accepting him finally.
Mr. Happy from The Mr. Men Show is the example of the sympathetic kind. While in the books, he was nearly always happy, in this show's version, there's been times, when he tends to hide other emotions behind a smile to keep other people's spirits up. There's been episodes to show that like "Lake", "Canned Goods", "Collecting", "Music", "Ships" and others.
The Bloopers Guy from Robot Chicken puts on an enthusiastic and chipper act, but considering he tries to commit suicide after every show, there is something wrong with him.
And let's not forget why Butters needed two years of therapy after his last tap dancing competition....
Kenny, known for laughing at innuendo, is most likely this as of "Mysterion Rises" because of the fact that no one remembers any of his deaths.
The titular Sponge Bob Square Pants. A few episodes indicate that behind that perpetual smile and always energetic and optimistic personality Spongebob isn't entirely sane. There have been episodes where he would go into Freak Out mode over little things or become stark raving mad if things don't go right for him, like if he encounters some problems at his job that he loves and the like.
There are no indications that Spongebob is sane. The flavor of his apparent insanity varies.
In "Selling Out", Mr. Krabs sells the Krusty Krab to a big corporation, where it becomes a cheesy family restaurant called "Krabby O'Monday's". The new manager, Carl, is perpetually cheerful, even when he's threatening Mr. Krabs to keep his nose out of the restaurant's business.
Carl: The less you know, Eugene, the better...
Ned Flanders of The Simpsons. He is perpetually happy, refusing to let anything get him down or bother him - even if it really should. Maude's funeral may be one of the few exceptions to this. One episode, which sees him suffer a nervous breakdown after trying to cope with the people of Springfield (very shoddily) rebuilding his house after it's been destroyed, implies that this is a result of him misinterpreting some advice given to him by the therapist who saw him for his anger management problems, when he was a teenager (and rebelling against his beatnik parents by being an angry, angry square).
Ned Flanders: Now calm down, Ned-dily-diddly-diddly-diddly... they did their best, shoddily-iddly-iddly-diddly... gotta be nice, hostily-iddly-diddly-iddly... Ah hell diddly-ding-dong-crap! Can't you morons do anything right?!
In fact, in one Halloween special, Ned Flanders becomes an Orwellian figure and makes sure everyone smiles - or go through "Re-Neducation", aka lobotomy.
An episode of The Fairly OddParents has Timmy Turner's dad become this as a result of Timmy travelling to the past to prevent him from winning a race and thus meeting Timmy's mom. This results in a dystopian future in which Dad becomes a Stepford Smiler to cope with his loss and forces everyone else to be one as well.
There is a hint of this lampshaded by Timmy's mom after being swapped into dad's body in one episode, where she mentioned that she had a sudden feeling of giving up on her dreams. This could mean Timmy's dad gave up on his old dreams sometime ago.
Also, in the FLARG episode when Timmy is trying to stop Mark from exploding:
Mark: Dude, what's with the face? It is happy, yet at the same time DISTURBING!
The titular Kung Fu Panda Po. It's implied that he has that cheerful front to hide his insecurities. Subverted because some of his smiles are genuine.
The title character of Hey Arnold!. Most people see him as a sweet, friendly little boy...however, he's really just hiding his sadness due to his parents being lost in the jungle and not seeing them in ages.
On Daria, perpetually happy Mr. O'Neill gets at least one moment of this: in "Is It Fall Yet?" Link tells him off, saying that he either doesn't really care about helping people or just "sucks" at it. (It's the latter.) After Link storms out Mr. O'Neill reassures himself that Link didn't really mean that, then goes back to his work, only to make a loud sobbing noise a moment later.
The Ice King from Adventure Time is shown to be extremely depressed in several episodes, yet is also a giggling Man Child quite a bit of the time.
Parry from Robot And Monster is always smiling, but only because his face gears are broken and he can't make any other expressions. In reality he's the most miserable character in the whole show.
Impulse from Young Justice is a Type A: he presents himself as a happy, peppy tourist from the future "with a broken rental car." In reality, he's an extremely serious and driven person from a Crapsack World (flashbacks imply he was enslaved at one point) who knowingly trapped himself in the past in order to change it.
Quagmire, the sex maniac that brags how he sleeps with every woman he meets, has a shell on him that no one but Brian knows about since he was the only person Quagmire told his secret to. The reason Quagmire sleeps with any woman he meets is to fill the void he has after he broke up with the love of his life years ago, Cheryl. Unless it is Cheryl, sex and women in general won't feel the same. Brian actually uses this weakness against Quagmire by dating Cheryl just to piss him off after Quagmire taught him the wrong things about love.