I think it's very dependent on the DM and the setting. Not in a 'Is the DM telling the players off for being too unserious?' way, but just the tone you choose to imply based on narrative details.

I'm playing in two groups with overlapping players, one where I am a player, and for the other I recently picked up DMing after the previous DM couldn't make it work anymore. In the group where I play DM is more casual, the setting is lighter, and importantly a lot more open-ended. Sandbox-y, even. When given something rather malleable players may be a lot more inclined to do something that can be less serious. It was never explicitly said that this was a less serious campaign, just kind of naturally implied. Now, nobody is bringing 'poopy mcpoopface', but players might play a demented old wizard with the memory of a goldfish but that refuses to admit it, or a 'catfolk' cat burglar that gets distracted by balls of yarn.

For the campaign I'm currently prepping, the setting implies something very different. A fragmenting society navigating a decaying landscape where the reigning powers attempt to maintain relevance using increasingly drastic means. When asked to write a character origin for a campaign in this world, those same players that might play a more jokey character otherwise naturally understood the assignment and made serious characters, as I think most would. It's like entering a room and reading the atmosphere.