Ugh, this sucks and I'm sorry you have to put up with this. I am not going to tell you to hide under a tent and massive layers and vertical lines that will give the "appearance" of being slimmer. I think you should just subtly–and stylishly–rub in her face that the body issues are hers and not your own and show up in a bold, bright look.
French Connection Silk Corsage Neck Full Silk Dress $198.25, ASOS
Like a pleated red dress! Pair it with some cute gold flat sandals and who the hell cares about what your mom says? Nothing in an empire waist either, just stick to figure-conscious silhouettes that have a slightly defined waist.
Midi Dress in Check Print $86.20, ASOS
Don't shy away from prints either!
If pants are more your steez, then why not get into some super bright skinny jeans or trousers with your favorite breezy top or blouse?
I know moms are important and all, but you just had a baby! That is amazing and in a way you should celebrate your body for being able to go through so many changes and bring you what I'm sure is a little bundle of joy. Who has time for bad energy these days? Not me, and definitely not you.