Being the only one who didn’t laugh at the mean joke can feel lonely. It’s still the right call.

Not joining in is visible in a way that joining in never is, and the silence gets noticed by everybody, including you. That loneliness is real, and it is also brief, unlike the version where you laughed along. More often than you would guess, at least one other person in the room was relieved.

No speech is required. A flat face and a change of subject does the whole job. Later, say something to whoever the joke was aimed at, because that is the part they will actually remember.