Well if "punk" means bad then yes it is! It's very hurtful to the other person and although you may think you're just picking on that person, you are more than likely causing them many years of trouble. Not cool.
It is punk to bully someone?
Yes because you are making someone else feel small and it's for nothing. You just want to go and push people around and think that you are big and bad. So yeah you are a punk if you are a bully.