Bottom.

1. The risers on the stairs are not visible.

2. The wear on the carpet goes right to the edge. That’s consistent with people stepping on it, not kicking it on the way up.

3. If it’s at the top, the mattress doesn’t appear to have any of its weight resting on a step. It could be so wedged in that it’s being held in place, and that it was wedged that way by someone awkwardly pushing at the ends of it in a way that wouldn’t seem to give them enough leverage to do that. But the obvious explanation is more likely, that it’s at the bottom of the stairs.