A mattress would be a "band-aid"...the same way that sleeping on insulation in lieu of a mattress would be.
It would provide some type of absorption/insulation...but not a complete solution.
I would find it kind of cumbersome and unruly positioning a couple of mattresses around me any time I wanted to record....but you might feel different about that.
People in the home rec world tend to "make do" with all kinds unusual solutions for a variety of reasons, instead of going with something more proven and commonly used....be it budget, convenience, inexperience...whatever.
In the end...do what you think is best, and what you can for your situation...and if you like the results, that's all that matters.
Got ya, I wouldn't want to sleep on insulation! Prices for high density stuff really isn't bad for a solution I can take with me anywhere. So, how do I begin thinking about what I need specifically for my room? I would like to get some experience tuning a room. I've walked around with an AT2020 with monitors on clapping, singing low notes etc, and it seems like the most dead sound I've gotten was in my closet with thermarests and sleeping bags piled up.
It was too small, so now it's all in a corner with HEAVY wool blanket draped over my PA speaker. I also figured the MDF in the PA speaker and a few amps heads I have would catch some bass, so they are in the corner as well, along with a couple of guitar boxes stuffed with foam and bubble wrap with blankets hanging on them. That's all behind my "mixing desk" which is actually just a philips hi fi stereo, and a USB speaker. I also mix in my living room on a pair of polk bookshelf monitors.
Sorry for the details, just figured I would let you know where I'm at here. I'm still working on the performance aspect of my gig right now, and just doing sketchbook types of recordings. I'm just trying to hone it a bit as I go.