Does the Pack n Play need a mattress?

Ma

I'm at first time mom and am thinking about getting the Graco Pack n Play for my newborn due in August. I was just wondering if any of you guys who had one and used it bought a mattress to put on top of the bassinet part?

I saw that Amazon had a few mattresses that fit the pack and play, but I wasnt sure if it was really necessary or not. I was thinking of just doing a sheet on top, but I want to do what is safest.

Thanks! 😊