+1 from me - mutt is the clear owner of these paths so write permission is reasonable and expected, plus the name dot-mutt also seems reasonable to collect these under.
+1 from me as well for auto-connectpersonal-files with write access to $HOME/.mutt, $HOME/.muttrc and $HOME/.config/mutt using the iface reference dot-mutt. As explained in the snap-configuration section of the project, those are the expected locations for mutt configuration files.