I don't know about mushrooms, that could look kind of weird as a background (although I have never seen it). I have seen GSP backgrounds (like fastrc) and Xenia backgrounds that looked really nice though!
Again, I have never seen anything like that, but I'm sure if done right and the wall is completely covered it would look just fine! It definitely can't hurt to try.
Good luck glueing them on! Shrooms and Xenia are the hardest things to glue imo. You need to put a rock leaning on the back with the shrooms and wait for them to spread to the wall. Thats how I'm doing my Xenia. Or cut off rubble were the shroom is attached and glue that and hopefully they'll start spreading. You also run your luck of the actual shrooms spreading. Yea some spread like crazy but some just won't. I've had some were they just start splitting and multiplying ans some forever that have never done it.