In my own practice I don't have any specific restrictions on position sleeping after a mommy makeover. Generally a position that is comfortable will not put any undue stress on tissues that have been operated upon. Nevertheless, if you have any doubts check with your own surgeon.
Typically, a mommy makeover includes a tummy tuck and most patients feel comfortable when sleeping in a slightly flexed, or bent position, while on their back. This can be done by placing pillows under your back and your knees. I advise my patients to do this for the first couple of days but gradually they will feel less tight and they are able to sleep flat on their back. Good luck!