The mare should be at a perfect weight all throughout pregnancy and while the foal is nursing. If she is not, she is not getting enough food.
Here is my mare just prior to foaling...324 Days here, she foaled at 337 Days.
Here she is with her 5 day old foal. She looks wonderful here.
Chilly with 3 week old Wrecker.
Chilly with 5 week old Wrecker.
And Chilly back in work with 7 week old Wrecker. (This is taken at a show)
Wrecker is about 8 weeks in this photo. And Chilly is actually OVER-weight. Not being fed hay right now, just 24/7 pasture, salt/mineral blocks and fresh water.
My mare has always been an easy keeper but with her pregnancy, I watched her weight extremely close. Once a broodmare goes downhill, it can be hard to get them back on track. It's a slippery slope and I didn't want to go down that road with my horse's nutrition.
There is NO excuse for skinny horses. Including "hard-keepers" and "lactating mares" It's just not an excuse. It's sheer laziness/lack of proper care on behalf of the owner if they have a horse that's a 1 or a 2 on the scale, let another poor broodmares.