I've recently dealt with this even before I got interested in IIFYM, it's hard to eat good in a situation where you don't have much control and it'd be wierd to speak up and say no to certain things.... Like my boyfriends mom would always make food that was delicious but so bad for you. Not to mention you have no idea what the macro content is!!!

My advice is basically postpone IIFYM specifically until you KNOW what the content of your food is going to be. It would just be to frustrating to guess. BUT in the meantime, below is my advice from my experiences (If eating "healthy" is still a struggle with your mom's food)

it may seem awkward and rude to talk about certain interests in diet change around the people that provide you the food, but for me I just basically had to sneak it in the conversation little by little very subtly, and gradually let them in on my preferences... So more and more each day my boyfriends mom would hear about something I learned about healthy foods or whatnot, and eventually with time she understood what my goals were, and she actually started making different foods altogether!! And picking healthier ingredients!! And pretty soon she picked up the same lifestyle that I wanted.

SO yeah best thing to do is be honest and talk to your mom about your wants, but if you can't do that like I couldn't, then subtly show her what you want. hope this helped!