There's 2 ways to look at it.
First, is a room with SOMETHING against the walls going to have less "echo" and sound at least a little more controlled than a totally empty room with rock solid walls? Yeah, it's going to do something.
But is it really going to be much better? Probably not because you're going to notice other problems and side effects from using mattresses as opposed to the proper materials like rigid frbreglass or some other similar material that will do the job properly.
The problem with using material that's not dense enough to absorb low frequencies is that you'll be reducing the mids and highs while the bass frequencies will be un-affected. Unfortunately, bass frequencies are 90% of the problem in 99.9% of rooms. So, what you're now doing is making the bass 100% of your problem. You'll have less echo, but you'll probably find the sound very dull, since you killed the mids and highs but not the bass. This will probably lead to the drummer hitting harder, the guitar player turning up, etc....which will then lead to hearing loss, fist fights, jail time, and eventual therapy.
Just something to think about. But, there's no reason not to try the mattresses if you have them there anyway.
