She does look like a balloon platy and they do tend to get pretty fat like that. You could try feeding her a little less but as long as she acts ok and eats fine, I think she's fine. Thawed & peeled frozen pea pieces are a nice treat for platys, also diced green beans and sometimes even broccolI or cucumber slices.
I bow to the wealth of experience in here. On closer inspection, all 3 paltys I raised from fry have distended bellies, so it makes sense that mum was ravaged by a ballon platy in the LFS. What a relief. Anything I need to know about ballon palty apart from their not being very attractive?