Just to throw a wrench in this whole thing (though I agree with pretty much everything that has been said), it bears pointing out that a horse with something like insulin resistance (not that the mare in question is at all IR, of course) could have fat pads and a cresty neck, but not be overly fat.
For instance, my mare is IR. Even when she's super muscular and very fit, her neck is still somewhat cresty and she still has fat pads behind her shoulders+along her spine. Her ribs are easily felt and she's a perfect weight for her body type, per the vet, but if you based it on fat pads, she's be deathly obese.
For her, I know that due to those fat pads (which disappear when she gets underweight), her ribs disappearing is my sign that she needs to loose. If I can see her ribs when she sneezes/coughs/stretches out at the trot, I know her weight is acceptable. If I stop being able to see a little rib at times and there begins to be a covering of fat over her ribs when I feel for them, I know she needs to cut back a little.
Fabio - 13 year old Arab/QH gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.