As long as she's not employed/paid by snapchat and trades on her account - no, not illegal at all - she's a private citizen making a public statement with no insider knowledge -- her ability to _affect_ the security is not material (the board or auditors of SNAP _might_ be on the hook if "celebrity disinterest" wasn't listed in the known risks of their last quarterly -- I mean, that seems like a reach, but I'm not a lawyer, judge, securities expert, or fully functioning human being...).

Ethically, it might be nice if she disclosed a position, but she's [presumably] not a financial analyst or advisor, so she's got no duty to disclose.

I mean, if you were president, you could make good money for yourself and your friends just telling them which way you were going to tweet about a company a few minutes before you do -- if you didn't care about the appearance or ethics of it...