With Quill mites they strip the feather and just leave the shaft. She doesn't look like she has any mites of any kind. If this is at the base of the neck on the back, it is possible others are aggressive toward her and they are chasing and grabbing at her feathers and pulling them out.
Northern Fowl mites are one of the most common of mites and you will see what looks like black dirt at the bases of the feathers. These are eggs. Her feather shafts look clean.
I think this is aggression from another bird. Take some goodies up for your birds and toss them around the run and watch carefully. If she is losing these feathers to aggression, you will definitely see it at treat time.
She is at the bottom of the pecking order...poor thing. That I know for sure. I would just write it off as aggression, but she is constantly preening and itching at herself. I did order an apron for her online to see if that helps. She almost does summer salts while she itches. Poor girl. With the treats, she's pretty good about grabbing her treat and running for the hills, lol. I'm just not sure, poor girl.