Regardless if she is underweight or not, I must say that it is very obvious she is being fed what she needs because there is a very healthy, beautiful shine on her coat. Her hair looks to be the correct thickness (not brittle and not coarse).
Unlike most horse people (except Ruger, he and I are on the same page, lol!), I actually like my horses slightly underweight. I like to be able to see the very top of the last three ribs. It gives them an athletic look. I do a lot of riding and toning on my horses though.. I feed them properly and their coats can attest to that. I just try to keep the bellies off of them by working them. Again.. my horses are slightly underweight but not neglected. =)
I think that mare looks fine. She has a really nice top line. The only thing I see that looks odd, is where her barrel meets her hip, it is dipped in a little there. But, I could very well be seeing this wrong because I had my head tipped to see the picture =) If you could get a good side shot like PeggySue asked, I could give ya a better opinion!
If you choose to put her on a fat suppliment (i wouldn't put her on a fat suppliment if she were mine) I would recommend one with a Higher "Fat" content, rather than a Higher "Protein" content. So you wont have to worry about her getting too rich of a diet and foundering, etc. =)
Again, im blinded by how beautiful that coat is, she is obviously being well fed (not overfed, but being fed a complete feeding... so she is getting vitamins for her coat, etc)