Some things are actually ridiculous though. Ridicule does provide a social purpose.
That doesn’t make it ok to bully a child or anyone else. But it is ok to point out that a belief or a behaviour (not the person) is ridiculous.
‘you are not a cat. Stop being silly.’ isn’t bullying or ridicule. It’s just a response to a child being silly. And it’s milder than the rebuke you’d get for plenty of other ridiculous behaviour at school.
It I started turning up at work in cat ears and insisting people view me as a cat, my employers would tell me stop being ridiculous and that I can play at being a cat in my own time if I like but not to behave inappropriately at work. And they’d be absolutely right to.