She does seem stunted. Sometimes if they have a bad case of coccidiosis when young it can damage their intestines, and they won't process feed as well.
Could have worms too, or at least had a heavy load at some point. A fecal is always a great idea.
If it were me, if you see any mites still moving, I'd spray her again. You're not going to overdose her. Did you spray the other chicken too? If not, I would.
I also would not feed greens, they can cause loose stools. Her chicken food and scrambled eggs are fine.
@Wyorp Rock I looked up that spray she used and it has something like 0.03% pyrethrin, although it does have good Amazon reviews.
If it didn't kill the mites, I would go get something else to try, but getting and keeping the mites off the bird is the #1 task here. And then start looking into...
Permethrin is extremely safe, it won't hurt her at all.
The only thing to keep in mind with permethrin is to keep it away from cats. It's fine for basically anything else.