Ah great, that put me down the right path. I did have the .info, and my retroarch.cfg for NES was pretty bare bones, but looking for other .cfg files I found this:
Turns out I had a really old config file under ~/.config/retroarch/config/FCEUmm/FCEUmm.cfg
Renaming that file so it wasn't loaded fixed the issue - now I can use save states in nes games again :)
Thanks!