Wicca is a religion that incorporates spell-casting, so presumably Billy chose the name because that is what he does.
He originally chose the name 'Asgardian', but felt it portrayed as more than human, which he is not, and changed it to 'Wiccan'.
Also, Billy is gay. Asgardian could become Ass-gardian by bully's and other "comedians". Therefore he was advised by his boyfriend to change his alias.