Not always, but it's good to rule out crowding and/or nutritional issues.
I'm curious, is there always a reason why a hen is a bully besides her personality? Not to hijack this thread but I have a hen who is mean to everyone and I've always wondered why, but can't figure it out.
Also good to define 'mean' by learning more about when/how the bullying presents.
Chickens peck each other all the time, it's a normal part of their communications, some folks don't understand the difference between normal pecking and bullying.