Yes, your poor baby is in serious needs of proper nurtrition. Stop feeding cracked corn-start with a diet of grower pellets/crumbles and lots of greens -get the grower feed from the feed store-tractor supply-they should have vitamins too
Along with the other good suggestions, I would get her some niacin. You can add a little brewer's yeast to her feed, but check the poultry vitamins to see if niacin is in them. Ducks need more niacin than chickens as they grow and poor growth, bad balance and crooked legs are the main result of not enough niacin. Of course too much isn't good either. So, if you get poultry vitamins with niacin in them, don't add brewer's yeast.
A little bit of hot pepper (cayenne) is a good tonic for poultry. Also a bit of raw liver, if she will eat it, is highly nutritious. Only give a small bit at a time though, it's nasty if it sits around!
I had this same issue with one of my muscovy ducklings, I believe it was a case of slow growth on him and the feather loss got worse as the siblings realized they enjoyed the taste of feathers
He was around half of the size of his sinlings for the first three months but I seperated him for a while and fed him layer pellets 20% protien and he got a lot better his leg got much better and he can finally run at now 4 months old
just keep her a hygienic environment and fresh water and a good diet and she will be fine within a blink of an eye.
She also seems to be a runner duck for whomever was wondering, am I correct? But Good luck I hope I helped