There's a violinist I play with in studio that closes his eyes and rocks his head back and forth whilst playing, as if he's 'lost in the music'. Looks ludicrous in my opinion.
I do get the whole 'live music is a performance' thing but it's not my cup of earl grey.
I am guilty of doing this occasionally especially if I am playing a long emotive piece. Honestly I think it makes you look better as if you really know about the music and your instrument ...
I think it just demonstrates how engrossed in the music they are, nothing wrong with letting yourself get carried away, don't really see why it would annoy anyone tbh lol, music is for the ears anyway.