I don't mind Jazz as a nickname. In general, the word feels neutral to me though I would guess, were I told I was about to meet Jazz Surname, that I was meeting a female. I think it could work for a guy if he were a very confident, outgoing person, but I imagine a young male Jazz would more likely run into teasing than would a young female. Sound-wise, I could see it as a nickname for
Jasper or
Jason but, if I wanted to use it, I'd probably spell it Jaz, relating more to the name component rather than the word.