If your mom likes doing it and she's not making you feel guilty, why should you? I agree, don't be guilty, be grateful. I remember what it was like having a newborn and a 2-year-old--exhausting! When DH went back to work after his 2-week paternity leave, I was tearing my hair out. I would have killed to have my mom there to help.
Sadly, my mom had late-stage dementia at the time (9 months ago) and has since passed away. When I read your description of your mom getting your DD up in the morning and putting her down for her nap, I think how wonderful those little grandma-granddaughter moments must be and how much your DD will cherish those memories, and I yearn for my children to have had something like that with my mom--for their sake and for hers. Oh, how she would have loved it! (And oh, how I would have loved the break!)
As you said, you do pretty much everything else for your daughter, and it's not like you're ceding her care to your mom. You're getting a lovely break here and there, they are sharing time together and building a relationship. It's great for everyone!