I could be wrong but it sounds like you isolated her on the eggs instead of letting her try to hatch with the flock. There are advantages and disadvantages both ways but that is one of the disadvantages of isolating a broody away from the flock, you often have to handle an integration. So if you did isolate her you need to think integration. If she was on a nest in the coop with the others, I’m not sure what is going on. I’ve never had that kind of a problem.
Is she really pulling out her own feathers? I have had hens molt while broody. Is she just undergoing a molt? When a chicken molts and lose all those feathers they can look a whole lot smaller than normal. They really haven’t lost any weight, just a lot of fluffy feathers. I have a molting hen that looks like that right now.
I’d check her for mites and lice. That can cause them to lose feathers. Check after dark in case it is roost mites. They hide during the day and attack after dark. They will run from light but you should see them scurrying away when you check. They normally like the vent area because of the moisture. Use a flashlight in the dark to check.
It doesn’t happen all the time but a broody hen will sometimes pull out some breast feathers, maybe to line the nest or maybe to get her skin closer to the eggs. I’m not exactly sure which and most of my broody hens don’t do that at all. Is that what you are seeing, the residual of that breast feather plucking?
How long was she broody? Before she even starts to lay, a hen builds up a fat reserve. It’s mostly in the pelvic area and is called a fat pad, but the fat builds up some all over the body. That fat is mostly what a hen lives off of when she goes broody. A broody hen can lose a fair amount of weight while on the nest but it’s just fat stored for that purpose. A hen that lost that fat and molts can really look skinny. It will take a while for her to regrow those feathers and rebuild that fat pad so she looks normal.
All this is nothing to worry about, except the possibility of mites or lice and you can treat those. How is she acting? I she running around and active like a normal chicken? If so I would not worry. But watch her and if she starts acting lethargic or just stands around fluffed up, she could be sick. Behavior like that is what would cause me to worry, not the other stuff.
