What about the mattress?

kitten
So my mom bought the crib for my baby but didn't mention anything about buying a mattress. I know it is a lot but do I ask her to buy the mattress as well? Or do I just put it on my registry and buy it myself if no one else does? 
It seems to be all she wants to buy is clothes for her grand baby. But I don't want to seem ungrateful for what she has bought.