Props to you for rescuing this RES.

Exactly how big is she (I don't see any long nails and that RES looks rather large, so I'm assuming you have a female))? Measure straight down the back from the scute behind the neck to the ones above the tail. I see the deformed marginal scutes (the ones that make up the rim) and yes, this is largely due to poor nutrition as well as an inadequate enclosure (incorrect lighting, etc.). The scutes also look in poor condition; they will look better after shedding, but unfortunately, the extent of the deformity will not be significantly reversed.
Any improvement will take a good diet, a good set-up that allows for ample basking under correct lighting, and TIME. Read up on the basics that your RES needs and make any necessary improvements to the set-up. I'd change the basking area since your RES finds it edible. Try gettting some cuttlebone (in the bird section), take off the hard backing, break off some head-size pices and let them float in the tank. Your RES is looking for more calcium in the diet ( a lack of which is part of the reason why the shell is the way it is), and this is an easy, inexpensive way to provide it.
What are you feeding her? At her size, pant matter should be making up a significant part of the diet (look at the list for what's good to give).
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-