Maturity Is Serious Business

This is a trope that doesn't appear much in fiction, being pretty much exclusive to Fandom. The basic idea is that the more seriously a character acts, or the more seriously a story takes itself, the more mature it is.

For example, if a Plucky Comic Relief character has a Let's Get Dangerous moment and spends a good portion of an episode, film, or book being deadly serious, odds are someone will hail this as the character "finally maturing." And when Plucky Comic Relief guy inevitably goes back to making lots of jokes, someone will complain that the writers are regressing the character. While rarely said outright, the implied idea behind this is that having a sense of humor is a sign of immaturity, and that a mature character will act with grim seriousness at all times. "This Is Reality" will always be invoked.

This same attitude can apply to stories as a whole, with comedies frequently seen as being less mature works than their dramatic counterparts, even if the comedy also has some dramatic moments.

Related to True Art Is Angsty and Comedy Ghetto. A leading cause of Darker and Edgier. Fans of this mindset generally want their show to be on the 'Serious' side of the Sliding Scale of Silliness Versus Seriousness all the time, regardless of whether or not it's appropriate.

On the other hand, overplaying this may lead to denunciation as being a mopey adolescent.

Film

 * In the film adaptation of Hairspray, cheerful Plucky Girl and The Pollyanna Tracy becomes very serious on learning that Negro Day was cancelled, meaning Seaweed's sister Little Inez can't dance on television despite practicing so hard. She risks her position as a dancer on the Corny Collins show to march with the black community the next day, saying that Inez deserves a chance as much as Tracy does to dance.

Newspaper Comics

 * Calvin and Hobbes had its first Growing the Beard arc when the two characters find an injured baby raccoon. While Calvin makes a few quips, he tells Hobbes to stand guard while he runs to his mother for help. He and Hobbes stay up all night worrying because the raccoon hasn't eaten anything, and cries the next day after his dad breaks the news gently that the raccoon died. Later, the two have a talk about death, why it is so scary and mysterious.
 * Curtis had an arc where Barry found a baby in the trash and started taking care of her. Usually the siblings bicker and are goofy. When Curtis finds out, he freaks out on realizing that "Missy" is sick and calls his parents for help. Both boys are somber as the doctors explain that Missy is a crack baby and she'll do better in the hospital as well as a foster family.

Western Animation

 * BoJack Horseman:
 * When this happens with Todd in "It's You," audience members know that BoJack has irreparably ruined his friendship with his roommate of several years. Todd is normally the Plucky Comic Relief of the show that engages in his own Get Rich Quick Schemes and Zany Schemes to achieve different goals. Then he fights with BoJack on learning the latter . While other BoJack misdeeds have also turned Todd against the horse, this was the Rage Breaking Point. Todd seriously tells BoJack to stop blaming the drugs or alcohol or his shitty childhood, and admit that he is a bad person, before leaving in disappointment.
 * In Teen Titans, there are several cases where Beast Boy stops being the Class Clown and becomes serious, shocking his teammates:
 * During "Betrayal", he actually has Slade on the ropes in the climax and starts mauling him after the latter reveals that Terra betrayed the Titans to him, and they're being attacked in Titans Tower. He also makes it clear to Terra that her apologies mean nothing because she sold their friends out to their worst enemy.
 * A similar thing happens in "Aftershock, Part Two" after Terra tried murdering Beast Boy as well as the other Titans in the first part before helping Slade take over the city. Beast Boy, after thrashing several Slade bots, takes it upon himself to track down Terra. On seeing Slade controlling her and Terra whines that she has no choice but to obey him, Beast Boy yells at her that she always had a choice, such as choosing to betray the Titans. This speech motivates her to fight back.
 * In "Homecoming, Part One" Beast Boy becomes completely serious after a distress signal from his former teammate Robotman comes to him. He finds Cliff stranded in the jungle, and leads the Titans to rescue the others, revealed to be the Doom Patrol. The other Titans are completely shocked that Beast Boy is completely focused on this mission, without any of his usual goofball antics.