It's not just some harmless fixation. This is a compulsive disorder, and it may have a small dietary component. Feathers are protein, and some chickens will eat them for this reason. However, it's been my experience that the majority of feather picking is a compulsive habit, originating in the brain, and difficult to treat.
If she just began this behavior, try giving her extra protein a few times a week and see if that affects the behavior. Over the years, though, I've had more luck installing pinless peepers to interrupt the behavior, and if caught early on, after a few months, the peepers can be removed and the behavior may not persist. It's worth a try.
