Spider-Man: When fans fell in love with Mary Jane Watson, many writers attempted to reconcile her devil-may-care party girl image from her first couple of years of existence by revealing that her party girl facade was something she came up with to cover up her screwed-up home life, which included a physically and mentally abusive father.
Empowered toys with this trope in respect to Mindfuck, who instead of putting on a reasonably happy face actively and regularly tinkered with her mind so she would be less psychotic than she would be otherwise reasonably happy. Of course if her posthumous confessions of being nearly suicidal are any indication, she ultimately played this trope straight.
Sistah Spooky wound up playing this trope tragically straight after Mindfuck dies, literally using magic to force any expression aside from utter despair onto her face.
In Gotham City Garage, Kara Gordon smiled constantly and pretended to be brimming with happiness as working for Lex Luthor. Reality is, she hated Luthor, her job and her oppressed, walled hometown and dreamed of escaping and finding freedom anywhere; but she never showed it.
Taken to downright depressing levels in issue #8 of the 2007 revival of The Brave and the Bold, a story starring The Flash and the Doom Patrol. Elasti-Girl is constantly smiling throughout, even when things look horrifically dire. (Master artist George Perez takes great care to give make her expression◊ completely identical every time: a bright grin with eyes that betray just a hint of utter desperation.) Flash explains to his freaked-out kids that she used to be a movie star and since gaining superpowers she feels like a freak of nature. She smiles because it's the only way she can still feel pretty, and she thinks it puts people at ease. Flash's daughter comments that it isn't working. It was later mentioned this was shortly after gaining her powers, and she eventually left this mentality.
In The Order, Becky Ryan constantly smiles, and it's often covering up her extremely deep insecurities. This can be seen when Henry Hellrung mentions that her psych profile mentions that she was seen as a suicide risk. Her reaction is to just smile at him for two panels and then insist that she's fine.
Winnowill of ElfQuest is smiling and regal, pseudo-friendly and aloof—when she's not getting her kicks torturing Strongbow or messing with other elves' heads.
Cat Grant, the Daily Planet's gossip columnist in the Superman comics is all smiles and sass, but she's very often braving it outwards to hide his private life problems, e.g. her abusive men, the death of her son, being fired from the Planet in the New 52, etc...
Deliberately invoked by X-23 in All-New X-Men: Laura is anything but well-adjusted, but tries to act as if she is for the sake of her boyfriend, Warren. She outright tells him her behavior over the first half of the series, which led to them breaking up in issue 4, was an attempt to "fake it until [she] makes it." Ironically, her efforts to be anything other than the X-23 of the past leads her to some very uncharacteristically reckless behavior that is what triggered the breakup in the first place.
Ken in The Doll House. He lives with a woman named Barbie, and while she has depth that is later revealed, he's a creepy Bateman-esque living Ken Doll.
Boss Smiley from World's End (and formerly of Prez) is, as one can deduce from his name, an exaggerated example.
In Transmetropolitan, one of the presidential candidates is known as 'The Smiler', and is shown as willing to kill just about anyone, including his wife, to improve his ratings.
Frank Einstein Jr. in Mini Monsters is a Depressed example: At first, he's portrayed as a happy leader and secure of himself and his abilities. However, In "The Perfeccionator" we know that Frank has a very low self-esteem, mostly because his physical appearance. He only acts like this because of his task of being the leader of the Frank's gang. In the end of the album, his self-confidence increases and begins to accept himself. Its subverted because some of his smiles are genuine.
He is always cracking jokes and acting as a class clown (particularly in the early Teen Titans comics), but later you find out that he's covering up for his sadness about all the people he's lost in his life and his fear of losing more (among other things).
Drift is normally annoyingly cheerful and upbeat in almost any situation, but in issue 22, he's asked if he's happy by Rewind. Drift's recorded reaction is to just silently bury his face in his hands. The fact that he spent most of the war as one of the Decepticon's worst mass-murderers, and isn't very much liked by his fellow Autobots even without that hanging over him does not help.
Also, Swerve, who on first glance is an annoying motormouth who is literally incapable of being quiet for more than a few seconds. Look deeper, and he's incredibly alone and miserable, with many of his friends dead in horrific circumstances, and knows he's not very well-liked.
Clarissa's family from the Clarissa comic series. Her mother tries to play a perfect 50s-style housewife, but blames Clarissa for her own molestation and wants her to just "admit" it's all her fault and pretend nothing's wrong, trying to bribe her into "behaving" with gifts and toys. Her father plays "perfect dad" while molesting Clarissa and manages to be insulted that she's terrified of him and can't see him without vomiting. One of her brothers is a neurotic mess that tries to pretend everything is normal in hopes that wishing it hard enough will make it so. Her other brother presents the image of the clean-cut, perfect son, but bullies and taunts Clarissa for daring to be traumatized by her experiences and "ruining things" by being a victim of molestation. The comic is the blackest of Black Comedy, finding humor in this ugly situation which is reality for too many children without pretending that it isn't truly horrible and vile.