A mummy tummy, aka still looking pregnant when one has lost most of the weight, isn't necessary due to the remaining weight but to separated abdominal muscles, called diastasis recti. That's a normal part of pregnancy as they have to separate to accommodate a growing uterus, however for quite a few these muscles don't come back together naturally. There are many reasons for that I won't go into (books have been written about it). The right exercise regime can help.
Whatever you do exercise wise, please DONT do abdominal crunches or any activity that causes the middle of the tummy to pouch out further - this makes the muscle separation wider.