Crib sheets sizes

So I picked out a crib mattress and my aunt wants to buy some cute sheets for it. I’m a first time mom and she has no kids so neither of us know nothing about it. The sheets are are 28 x 52 x 8. The mattress I want is 28 x 52 x 5.5! She’s trying to tell me that the sheets will be too big and that we need to find a smaller size but I feel like you want the sheet depth to be a little bigger right??? She things 2 2/12 inches is a lot. But I think it will be fine. Please I need advice.