Honestly, I really don't think the victim matters. A bully just targets someone. They can be someone popular, or unpopular. I think it is all about the bully. Sure, some kids are easier targets. Being very different from your peers makes you an easier target. But sometimes you don't have to be very different to be targeted.
I think a lot of it falls back on the parents. I know that if I found out that my kids was a bully, there would be some VERY serious consequences. And I would be proactive with checking up with the school to make sure all bullying had stopped. For majority of cases, the parents probably just don't care. Not all cases, but majority.