I've done a lot of wearable art, mostly jackets. I've learned that flannel often works well, but mostly I don't put any "batting" in them. I cut the pieces and quilt them to the backing on each piece and then put them together. They come out much nicer.
Was the fabric heavy or stiff to begin with? If it was, there's probably not much you can do about the stiffness since it didn't wash out.