Um, no, I think I’m fairly safe in saying, “Heating up food in a microwave doesn’t take away the nutrients.” Does heating food up in a pot on the stove, or in the oven, take away its nutrients? No.
The thing about uncooked food being more nutritious has to do mainly with fruits and vegetables. When you cook fruits and vegetables in a pot of water, a lot of the vitamins go into the water, and if you dump the cooking water down the drain, you of course have just dumped all the vitamins down the drain.
However, AFAIK raw meat is just as nutritious as cooked meat. Actually, cooking meat may even make it MORE nutritious, for example, with a really tough pot roast. Cooking it tenderizes it so your digestive tract doesn’t have to do so much work. Ditto for fruits and vegetables, if you can steam them so the vitamins stay in.
Potatoes are very difficult to digest in the raw state. Raw bread dough is also difficult to digest. Ever tried to eat under-cooked bread (because you didn’t want to hurt the baker’s feelings )and got a bellyache?
And cheese and tomato sauce remain pretty much the same substances whether you’ve warmed them up or not.
I understand where your mom is coming from, but, eh, sorry, she’s wrong.