The mattress is stuck too tight to pick it up but maybe you should try to cut a piece from it.
This is the low-spoiler guide for "How to pick up the mattress" puzzle in Beyond the Edge of Owlsgard. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.