Personally, I don't think it's the niceness that brings on the bullies. My dd is always very nice. I've watched her work and was shocked at how she could turn around an obvious attack and then play with the kids as equals.
That ability does not always (or often) follow from being someone who is always nice.
Being nice, even in the face of an insult or other testing of the waters by a soon-to-be-bully, leads to trouble when the person lodging the insult then decides that she can get away with it. Hey, I just needled this girl, and she didn't run and tell the teacher. Maybe I ought to try harder.
I'm told DD has never lodged an unkind word at another individual at school. Yet, as a rules follower that is also rather socially awkward, she's been subject to systematic bullying, both physical and emotional.
We are working on being less of a doormat and having a bit more social smarts, but we struggle against the thought that she will lose her instinct always be kind to others.
DS on the other hand, is also consistently kind to others, but when kids try to insult him, he shows some rough edges. So far, it seems enough to prevent the other kids from prodding further.