I agree, from the front, she looks to be in good weight.
There's a bunch of us on this forum with horses that have serious metabolic issues. We have gone from Liking them Plump to attempting to keep our horses just into the ribby side.
I hope this link opens for you. It's a link that shows horse owners how to measure their horse's Body Condition Score.
Stockier, raw-boned horses will naturally carry more weight than a slender athletic built horse of the same height, so this body scoring method is a better tool than just using weight alone
It's by a vet from the University of Kentucky. While Dr. Coleman likes to see most horses in a 5 - 6 category, as the owner to two horses with insulin issues, I prefer to have them lower. I am succeeding in keeping one at a low 5, the other is almost a six. What's Your Horse's Body Condition Score? | Video | TheHorse.com
There are also some excellent links on this page that talk about health issues that can result from horses being over weight.
You can also Google "scoring a horse's body condition and get some credible written hits, that you can print off, if you'd like.
Hopefully the barn owner isn't feeding sweet feed. Rice bran is great to add cool energy fat calories, as long as it's calcium fortified. Rice bran won't amp a horse up.
Grass hay is great
I'll bet when you post a side-view of your mare, none of us will think she's skinny