A Theatre trope, but could be applied to other media.
People who are attempting to get a big break with a musical that they are involved in, usually in a writing capacity, are often advised that they should do something funny, or light at the very least, because Serious Business doesn't sell without a creator's reputation to give people an incentive to go.
Contrast True Art Is Angsty, which reigns supreme in most other media (including non-musical theatre).
Compare What Do You Mean, It's for Kids?, when material aimed at children is criticised for being too dark.
- Joseph and the Amazing Technicolor Dreamcoat: Even though it was originally a school play, it did give Andrew Lloyd Webber and Tim Rice a springboard that they used when writing Jesus Christ Superstar. That success lay in rejuvenating old material, although JCS was arguably more accessible than funny, as Joseph was.
- Dance a Little Closer, Alan Jay Lerner's final musical, was a Setting Update of the popular 1936 play Idiot's Delight...meaning it was a musical Dramedy set on the eve of World War III (or, as the script described it, "the avoidable future") rather than the then-brewing World War II. As the Cold War was getting rather hot in 1983, the premise was a massive turnoff to audiences and it only lasted one official performance.
- Les Misérables: If the name didn't give it away: Thenadiers aside, this is probably the number one aversion. The professional reviews of its debut bashed it for being far too gloomy, claiming It Will Never Catch On. Word of mouth from the rest of the audience, however...
- Elisabeth: Even the sweet, innocent songs at the beginning ("Wie Du", for example) have horribly dark reprises later. To think of it that way, there are no happy or lighthearted songs in the entire musical the happiest moment is when the title character dies.
- Inverted in-universe in Finding Neverland: James' producer thinks theater-goers will pay to see only serious works, and thinks the idea of a fanciful play about children is ridiculous. The aversion is somewhat justified, in that James has to bring orphans to the play to laugh at it in order to get the adults to be receptive.
- Next to Normal: Drugs are an integral part of the plotline, the protagonist has spent years suffering from a mental illness, and the only member of her family who isn't affected by this is the hallucinatory version of someone who's been dead for years.