You ask for a critique, but without any reference to health. Almost impossible really, since I feel her health might have a lot to do with her appearance.
Others here, have given you lots of very good advice on feeding this filly. Get her off fescue grass/hay asap. It just isn't good and can be even dangerous. Do a google search and you will get much information.
I see she is 2 years old. How tall is she? Do you know anything about her sire and dam?
Only going by the pictures, it is difficult to tell a whole lot, since she has a winter coat. She might be more lean than she looks, under all that coat. If she were in good condition, I don't think she is long backed. I do think she might be a bit straight in the rear. That might look better if she were more muscled.
Her pasterns look long, but that might be because of hoof length, which I can't really see very well. She definitely toes out on all four feet. She is very narrow in body. Her neck is short and hollow. That tells me at some time, she has been severely malnourished. It will improve with good condition. Can't do much about her head, but once her neck improves, her head will not appear so large. Actually, I think her head is not too large for her body.
She does appear goose-rumped and bum high, but that could be because of her age at this point. With good care, she could become a cute little pony. I tend to think she will never become horse size. Certainly do not allow anyone to ride her, until she is in perfect condition and well over 3 years old. She needs time to recover from the obvious poor care she has had in the past.
Is there any chance this filly is bred? Just asking.
Here is a pic of one of my daughter's little Gypsies, at 2 years and 1 month. While I don't expect your filly to have the same type bone, your little girl should end up looking in as good a condition as this one, with good care and proper feeding.