Worse than being merely off-putting, I think it actively hurts, personally and professionally, when your don't get as excited over every little thing as your peers. People take it as a sign of indifference, and judge you for not being a "team player" or other such nonsense.
I long ago learned that it's always best to keep your mouth shut rather than saying negative things in public (though it can certainly be a lot less fun). But the little trap that no one warns you about is that it's possible to be perceived as negative simply by being less enthusiastic than baseline. And it drives me nuts when people put me into positions where my only option is to feign enthusiasm, or to say something negative:
Rude Person: "isn't Yanni just the most brilliant musician ever? And how about that Kenny G -- jazz will never be the same, right?"
Me: "um. indeed."
This kind of stuff happens all the time, and it never gets any easier to navigate.