Great question, and @robepisc beat me to it. I have added the following text to the store description in lieu of not having access to a man page or similar.
“This is a confined snap, so is unable to read the ~/.muttrc. So place your configuration in ~/snap/mutt/common/.muttrc where the snap will be able to access it.”
The snap also bundles vim/nano/emacs so you can configure the editor to use whichever of those editors you prefer. I appreciate it’s not optimal to have all of them if you only use one, but I am not sure what other option we have here. The snap can’t access binaries outside the snap (due to confinement) and I’m not going to choose only one editor and have everyone dogpile me for choosing the ‘wrong’ one
The binary path to the editor will of course need to be specified in your
.muttrc which will be either
$SNAP/usr/bin/emacs-nox or similar. Happy mutting!