From Toy Story 2, the Barbie dolls, whose gleaming smiles never falter, not even once. In the Hilarious Outtakes, one of them was complaining about how her face hurts from all the smiling.
Anna from Frozen, at least at the beginning of the movie. She is constantly cheerful and friendly despite her loneliness and desperation for someone who loves her especially after being abandoned by her own sister, although she is often seen briefly frowning in response to something negative before pasting on a smile. Over time though, she eventually becomes a genuinely happy person by the end of the movie as she makes new friends and finds true love.
Unikitty from The LEGO Movie is unstable; her bubbly personality hides serious anger issues, and when she finally snaps...
Riley absolutely refuses to ever be sad as she thinks that it's for her parents' sake. This has the unfortunate side effect of having Joy in control all the time and keeping Sadness out of a job. It also makes her a bit emotionally unstable, because Joy can't always handle every situation by herself she needs Sadness to signal that something is wrong and should be addressed.
Coraline has the Other Wybie, who was literally made to smile despite his actual emotions. So much so, that when he expresses his disapproval through a frown, the Other Mother sews his lips into a perma-grin. The stitches are cut, but not before we see what the Joker might have looked like as a child.
The seemingly-kindly Lotso Huggin' Bear is actually a bitter old man (well, plush bear) who runs Sunnyside Daycare like a cruel prison warden, and has never gotten over being abandoned by his former owner.
Chuckles in Toy Mode, additionally falls under the Sad Clown trope.
The Chatter Telephone. Don't let his Fisher Price sticker-happy smile fool you, he's quite tortured inside.
Mixed Gender Examples
"Bonjour! Bonjour!" The villagers in Disney's Beauty and the Beast sure seem to be a happy, friendly bunch...but they think Belle is a Cloud Cuckoolander in part because she's always reading and seems to be the only person in town who doesn't think anything of Villain with Good Publicity Gaston, whom everyone loves to the point that they help set up a wedding for him when he hasn't even proposed to her yet (and this doesn't even get into the cruelties heaped upon her poor dad, Maurice). By the end, this shallow bunch has become a Torches and Pitchforks mob headed off to kill the Beast because "we don't like / what we don't / understand / in fact it scares us / and this monster is mysterious at least..."