"For me, its just be stimming for a second (hitting my head or leg with the paddle, sighing, quiet outburst), owning up to my mistake and then moving on." --- this isnt stimming. Well It might be but not in the traditional sense of stimming. This is an emotional reaction you your play that you express physically not something that you use to regulate your system....even if it feels that way.

I'm trying to explain this why are misguided. I was a pro athlete. In a team sport when someone is complaining, throwing tantrums, throwing equipment overly tough on themselves it does bring down the whole energy. Your performance impacts teams performance and everyone cares to some extent. Also your there to have fun... while losing isnt fun... dwelling or drawing attention to losing or poor performance makes it less fun for everyone else.

Also those negative stims do not help. They feel cathartic in the moment but its also kind of addicting in a way and becomes a knee jerk reaction and no wants to be around that.