Adventure Time Marceline is a prominent example. She has acted like a mischievous prankster, although on the inside she's very lonely, lost and betrayed by a majority of her friends, and confessed it in her diary in a darker manner.
Marceline: My vampire eyes see only blood... red... skies; Blood red skies make tears inside that I always hide.
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.
The brainwashed Joo Dees (shares initials with Jane Doe). Mentally, they're practically Stepford Wives. Of the government. The worst part about these women? You get so used to those creepy smiles that it's almost scary to not see them.
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 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 controlling 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.
Numbuh 3: Soooo, what we gonna do tomorrow Numbuh 1? Numbuh 1: Ahh, yeah, I'm not going to be here Numbuh 3. Numbuh 3: Oh... And what about the day after that?. Numbuh 1: Oh, let me explain Kuki... Numbuh 3: Oh, I know you're leaving, silly. What do you think I am, an airhead or something? Numbuh 1: Never for a second.
Foster's Home for Imaginary Friends: The episodie "Berry Scary" has the titular character, Berry. 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:
Dexter Douglass' mother from Freakazoid! appears to be Empty. The writers credited this to Tress MacNille's delivery, noting how she could take any of her lines and still sound unusually chipper regardless of what was being said.
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."
Helga's older sister Olga is Depressed. 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."
In Home Movies, the rather joyful character Melissa was later revealed to be Depressed, 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.
Heloise of Jimmy Two-Shoes is Unstable. While she does have a cute appearance and tends to act sweet around her crush Jimmy, she is an incredibly cruel and sadistic Enfant Terrible with a Hair-Trigger Temper who enjoys torturing people, destroying things, and conducting dangerous experiments. Even Jimmy isn't safe from her wrath.
Bloberta Puppington, Orel's mother, 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.
Aunt Bogunda from Mr. Bogus appears to be the Unstable version 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".
While Pinkie Pie's smiles are usually sincere, she becomes majorly Unstable in the episode "Party of One". And then becomes Depressed in the episode "Pinkie Pride".
In "Princess Twilight Sparkle, Part 1", Princess Celestia reveals to Twilight that while she put on a brave face for her subjects, to her, the Summer Sun Celebration was just a bitter reminder that she had had to banish her own sister after her FaceHeel Turn. Since Celestia is a Slave to PR, she has to put a smile on her face at every occasion, no matter how much she dislikes it. "A Royal Problem" shows how much this facade exhausts her.
When Luna has to take on Celestia's duties in "A Royal Problem", she makes the mistake of plastering on a much larger smile, bordering on the "Unstable" variant, and ends up slipping into a scowl out of fatigue right as her picture is taken for a school fundraiser. This costs the students a field trip, an act that Lunakicks herself for for much of what remains of the episode, to the point where she has a nightmare that night with the students singing "That smile's too wide. It's obviously not real." as she practices the fake smile in front of a mirror.
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.
In "Twilight's Kingdom, Part 2", when Twilight tries to reassure Spike that everything is fine, she forces a big, insincere grin to her face.
The most extreme example yet is in "The Cutie Map", listed under "mixed gender" below.
Virginia Wolfe from Rocko's Modern Life. She (and Heffer) act the most cheerful out of the entire family, but she also has a nasty twitch. Best shown in this line from "Who's For Dinner?":
Virginia (to Cindy) Nothing's wrong, dear. *twitches*
She-Ra and the Princesses of Power: Scorpia is gradually revealed to be this. While she's been generally chipper and upbeat throughout earlier seasons, apart from some moments of vulnerability regarding her crush on Catra in season 2, the fourth season shows that her optimism about being part of the Horde is held up with a tissue-thin structure of nonsensical rationalisations that seemingly exist solely to delay her having to deal with the objectively horrible situation she's been left in. The Horde took her people's land, runestone, and children - including her - and she's sure it was an entirely peaceful transaction that definitely wasn't carried out at gunpoint. And anyway, her grandfather must have known, even when Scorpia was a literal baby, that she'd be happier as a soldier than a princess. And she is happier, and that's great, because the Black Garnet probably wouldn't accept her anyway, and the other princesses were always jerks about her family so who needs 'em. And as for Catra, well, she's sure that Catra is the best possible friend and will start becoming a better person any day now. When Emily challenges that last rationalisation by reminding Scorpia of what her genuine friendship with Entrapta was like, it doesn't take long for the whole thing to come crashing down, leading to Scorpia making a HeelFace Turn, finding friendship and acceptance with the Rebellion, and generally losing the "Stepford" part.
Scorpia: Push down doubts and insecurities...[sigh] Check. I am brave, strong, loyal, and I give great hugs, and I'm going to be the best friend that I can be!
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.
"Moaning Lisa" 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, she'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 Jerkass 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.
Amethyst of Steven Universe definitely seems to be one of these with the reveal that she was the product of a horrible plan by the Homeworld Gems to make an army and leave Earth uninhabitable. She makes it very clear in "On the Run" that she hates herself and believes she's nothing but a mistake because of her origins—something she's been hiding for thousands of years (though she might have told Rose how she really felt).
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. A later Bloopers special dedicated to his life shows why: he had an alcoholic and abusive father who beat him as a young boy for expressing feelings for Boy George (as, given his young age he thought he was genuinely a "pretty lady"), a trip to Mexico where he drank water that gave him such bad diarrhea, that his anus is still badly scarred and his "first time" with a fat girl got her pregnant and the resulting child, now thirteen-years-old, is pregnant herself...and later miscarried. No wonder he's so depressed.
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.
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 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 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. In "Future Lost", Timmy's dad all but stated he did. When Timmy asked how long that was, his dad merely replied: "How old are you?".
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.
In Daria, perpetually happy Mr. O'Neil gets at least two moments 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. (To be fair, 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. He also has a miniature breakdown in "The F Word," when he believes that his assignment has emotionally scarred his students.
Foster's Home for Imaginary Friends: Although Wilt is genuinely cheerful and happy with his life in Foster's, the one-hour special "Free Wilt Hunting" reveals that Wilt abandoned his creator after losing a basketball game years ago due to a Friend or Idol Decision and in the beginning of the episode he was smiling in a very creepy way over his guilt at the five-year reunion between imaginary friends and their creators (which his creator never showed up for). It is implied that he may have been doing this for years.
Then there's the episode "When There's A Wilt, There's A Wilt" where Wilt keeps doing favors for people due to his inability to say no. It reaches to the point where he starts traveling everywhere doing more things for people and ultimately misses the basketball game he was going to watch. His overall behavior says this trope directly.
The Ice King is shown to be extremely depressed in several episodes, yet is also a giggling Manchild quite a bit of the time. This gets more focus as his genuinely tragic backstory is revealed, as is the fact that his crown is basically an Artifact of Doom that has driven him insane and partially wiped his mind... and yet some piece of the good man he once was is still trapped inside the Ice King and has not quite been destroyed by the crown's influence.
Main character Finn is another one. He openly acknowledges that he represses many of his negative feelings and traumas, (he calls this process "putting it in the vault") and tries to keep from showing his various disappointments and inner sadness to others.
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.
Tangled: The Series: Rapunzel occasionally becomes this, whether due to her responsibilities to the kingdom or some traumatic event that she can't process (of which there are plenty in this series). Raps' general response to anything frustrating is to just bury it deep inside her until it becomes impossible to ignore, while putting on a brave face and acting like nothing's wrong. Perhaps the darkest example is in the 12th episode of the 3rd Season, where Rapunzel sings that she's "The Girl who has Everything" and has fun painting her room with drawings of her past exploits... and Cassandra is not in a single one of them.
Rapunzel: Somehow that's me, the girl who has everything Well, practically everything. I'm sure there's more everything. For now, I've got everything, And everything's fine!
In an alternate world from Justice League, Superman assumes totalitarian control over the world, and takes the time to lobotomize all the inmates of Arkham Asylum. The Joker's Slasher Smile is turned into a Stepford one. Much less dangerous.... But just as creepy.
The eponymous Steven Universe. While legitimately optimistic about life, he is weighted down by the grandiose legacy of mother, who is considered to be one of the kindest (by her comrades) and most powerful (by everyone) warriors to ever exist. This is combined with the fact that she gave up her life to allow him to exist as a Half-Human Hybrid, leading him to also admit that he feels as though the others blame him for her death. Steven Universe: Futureexamines this further, showing the stress of Steven's adventurous and dangerous childhood and all the emotional baggage that came with his mother's legacy has actually given him symptoms of PTSD.
Class of 3000: Philly Phil, despite his rathereccentric personality, is revealed in Season 2 to have serious self-esteem issues, going as far as to transfer to another school after feeling unwanted by his friends.
Also, as revealed in the final episodes, Numbuh 3. Which shouldn't be that surprising, considering her family...
My Little Pony: Friendship Is Magic: An entire Town with a Dark Secret spends most of the time smiling in a way that Pinkie Pie, being a smile expert, doesn't trust for a minute. They actually are genuinely friendly and even happy, in a way, but it's in no way worth it for what they've had to give up, nor is their happiness actually natural. Starlight Glimmer made the townsponies give up their real Cutie Marks, claiming that it was the only way they could all be "equal" and experience true friendship. Any pony who thinks Cutie Marks are good is locked in a small room where a recording of Starlight's voice drones on and on about how bad it is to be unique until they say that they agree with her.
Pinkie Pie: I know smiles, and those smiles? They're just not right!
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.
Played straight and zig-zagged with Kaeloo, who is somewhere between Depressed and Unstable. Time to time, her bottled up emotions actually change her physically into a hulking monster, the actions of whom seem beyond her conscious control. The falsity of her "cheerful" nature is often highlighted when she pushes back signs of anger with a huge, obviously forced, painful-looking smile. This counts as a mixed-gender example due to the canonical confirmation of the character being a Hermaphrodite.
The central theme of Moral Orel. All the peppy, upbeat 1950's stereotypes of townspeople are repressed, abused, and liable to snap at any given moment.
In "Miracle on Evergreen Terrace", The whole Simpsons family does this when The news visits their house right after Bart confesses that he accidentally burned the Christmas presents in an attempt to hide the truth until they can figure out how to handle it. Unfortunately, the snow thaws at that point.
Adrien Agreste from Miraculous Ladybug is a big example of this. Being a motherless Lonely Rich Kid, with a father who is overprotective, highly demanding and emotionally distant, Adrien holds this facade for most of the time. So far he's only dropped it in front of Nino, Plagg and Marinette. In the webisodes, he says that he only feels truly like himself when he's Chat Noir, meaning his smiles when in his super hero persona are real ones.
'The Evillustrator' heavily implies that Sabrina fits this trope as well.
This happens to the monks in the Xiaolin Showdown episode "Hear Some Evil, See Some Evil". After their teamwork started to become rocky due to being angry at each other's insults (which they heard through a mind reading device, no less), Master Fung suggests them to open their minds and thank each other for their insults. They pretend to do that, laugh it off and hug each other with a big smile on their faces, but when Master Fung isn't looking at them, they glare at each other.