It’s not quite that simple, not with anything. But even if it was, making snarky comments about someone else’s music (when it’s not being inflicted on everyone else) isn’t appropriate.
They can if they want to. Who cares? Whose business is it but theirs? Better to think it is “extremely awesome” than to censor what they’re playing because some other jerk can’t manage to keep their yap shut and makes snide comments as they pass by. (Which is, after all, part of what this OP is about.)
They don’t have to, but they can if they want to, as long as they aren’t using that belief as an excuse to inflict it on anyone else for any length of time.
Look—it’s kind of like believing your wife or your kid is the most awesome person on the planet. As long as you don’t expect everyone else to agree, and as long as you don’t put down anyone else’s wife or kid, who really cares? In your own little universe, you get to say what is awesome and what isn’t.