I can be goofy, but mostly I am a serious person. Even when I'm not feeling "serious" I come across as though I am, because that is my face and my default personality. Sometimes I brush that off and nudge myself in a more fun, outgoing direction, but sometimes I don't want to. Maybe I am too tired or maybe that is just the mood I am in. My point here is that the choice is up to ME. Don't be offended, I'm not "pissed off" unless I give you more reason to think so.
I am not Too Serious. I am exactly who I can and want to be. You do not get to determine what parts of me are too much or too little. Listen, it has taken me a long time to become okay (more okay) with my own personality. Yes, if you are doing something that annoys me when you think it is "just being goofy," you should stop doing that thing and respect my boundaries. That doesn't mean I am trying to shoot down your personality, it means you don't get to override my needs in order to fulfill your own. Go be goofy with the others if I am not entertaining you properly.
Repeat: You don't get to override my needs in order to fulfill your own. Wow, I want to have the guts to say that more often.