A mattress topper (basically, a piece of foam) is often appreciated by students, but I have never heard of an extra mattress on a ‘what to bring list’!
Is it Vassar your daughter will be attending? (knowing which college makes it easier to give specific info).
In general, the list you get from the college will be long and full of options (is there any hope your child will actually iron? what about Wrinkle Release?). Most essentials are usually on sale on campus, but with less selection and higher prices) or at least locally. I would not count on there being a vault or a safe in the room.