It's actually not at all bad for horse's metabolism to have regular 'bad seasons' & use up their fat stores/lose weight, but yes, if the bay's put on a fair bit & still looks like that, she probably was 'skinny' after winter & lost too much. Yes, slow weight gain is best.
Agree the appy is certainly not underweight - perhaps too heavy, but hard to tell from those pics & with the big belly, which I agree with other's questions/comments about. The bay doesn't look skinny at all, but does look possibly underweight still. Just because you can see their ribs, especially through a summer coat doesn't necessarily mean they're under weight though & the angular look of her sacro, hip bones & rump could be due to body/muscle probs rather than 'condition'. She certainly looks well padded around her shoulders, so perhaps it's just where she keeps her 'groceries'.
Have they been wormed *effectively*? Have they had their teeth done recently? Any possiblity of ulcers inhibiting bay's health? What are you feeding & do they get nutritional supplementation?