You think it's a problem if I tell my roommate to buy us a new mattress?
That depends, is there a problem with the current mattress you have?
No we just want a new mattress
A new mattress can help improve sleep, but you shouldn't expect someone else to buy it for you. You said there is nothing actually wrong with the current one. If this is correct, it's kind of an unnecessary yet fairly expensive purchase to ask of someone.
Haven't you ever wanted something nice?
Of course I wanted something nice in my life. But I provided those amenities for myself. You shouldn't expect others to provide for you.