She's not looking healthy, that's for sure. You can see her ribs and spine so I'd say she needs some help. It could be that she has a medical issue too though, ulcers etc. or it could also be that the foal she is carrying is taking all of the nutrients. Horses are tricky creatures. My daughter's rescue horse looked much like this when we first got him. He gained quickly but lost it again and its been a real battle to figure out what to feed him to keep the weight on. He's doing fine but I'd like to see him gain some more.
Let's hope this mare sells and the new owner knows what they are doing.