I would ask your mom what SHE wants to do.
My MIL watches our son two days a week (daycare the other 3) and she recently had a hysterectomy and couldn't physically pick up my son or chase after him. We arranged a different carer while she was recovering and then asked her is she still wanted to look after him for the two days or if we should enroll him in daycare 5 days a week. She asked to keep him for those two days.
She still can't pick him up or chase after him that well, but they do lots of fun things together - feed the ducks, painting, water table play, puzzles, books, etc.
I think it is great bonding time for your mom and daughter, as long as your mom is happy to keep doing it. I have a feeling your questioning whether or not your daughter is being stimulated enough in your mom's care. If she is a loving, kind caring grandparent and happy to watch your daughter, I think your current situation is great.
If you feel your daughter needs more activities you and hubby can do them with her Sat-Mon.