She's feeding the foal, not herself. Keep up the rations post weaning and gradually bring her back into work - top line is muscle, not fat and a program of gradually increasing walk/trot on the lunge for a couple of weeks followed by riding should see her back to normal within 6 months.
I agree that feeding the foal is tough on the mare.... My mares get up to 6 lbs of ration balancer a day when they have a foal by their side, and sometimes I supplement that with other feed.
The more babies a mare has the less topline she's going to have - their whole body changes because the only muscles they use on a regular basis are the ones supporting an ever-growing fetus.
In my opinion, age, number of foals, and quality of turn-out (i. e. hills and lots of room) affect the looks of a broodmare.
Siegi Belz www.stalleuropa.com
2007 KWPN-NA Breeder of the Year
Dutch Warmbloods Made in the U. S. A.
If your mare has her ribs covered, but is lacking on her topline, it's been my experience that she needs more protein in her diet. Ribs lacking=needs more calories. Top line lacking=needs more protein. If she's an easy keeper you can add a ration balancer. MOst of them, IIRC, are 30% protein. I usually just add alfalfa hay to their diet, as much as they will tolerate. I have one mare, who has had 7 foals, that I have to really keep a close eye on. If she's not getting enough protein (in the form of alfalfa hay), she drops topline FAST.
Chestnut Run Stable www.Zeltt.com
Standing "Tiz Brian" at Stud, 16.1 h bay TB by Tiznow
I agree that topline loss can be effected by too little protein in the mare's diet. However, I also agree with Siegi that multiple pregnancies can change a mare's topline fairly permanently. I also have some mares that never seem to lose topline and others that lose topline as soon as they are pregnant. The second category takes a long time to recondition if going back under saddle after a pregnancy.