Also going a bit off topic here, but I take issue with the notion that kids are faking mental disorders for attention.
Maybe some of them are, but I can see people actually having mental illnesses and wanting to talk about it, but not being able to bring themselves to be serious since they worry about being 'whiny' or 'angsty', so they decide to opt for 'cute and quirky' instead.
The point is if they talk about their issues in a way that's entertaining to others, they're not a burden because they're giving other people something positive in exchange for the attention and validation they're seeking.
And I don't think that 'cheapens' the experience of people who are 'actually' mentally ill; if people choose to call bullshit on someone who claims to be mentally ill, I don't think it's the fakers' fault for 'making' them distrustful of such claims. If you distrust the claim, that's fine: just ignore them. "Calling their bluff" will do nothing if they're lying, hurt them if they're telling the truth and make a significant portion of mentally ill onlookers afraid to talk about their experiences in both cases.
Edit: You did also bring up "tricking vulnerable kids into thinking they do have a mental disability" - I haven't seen much evidence that healthy people can be convinced they have a mental disability from mere exposure to random internet people claiming to have said disability, at least as anything more than a passing thought that they'll get over in a week or two, after which they'll move on with their lives like normal. But I'm open to being shown otherwise
(That's not to defend TikTok; I haven't been on there much myself and basically know nothing so I'm not going to make any claims for or against it as a platform :P)