I am currently using imap notes on my fastmail account. Does aerc support editing those notes?
If no, it would have been nice...
If you mean this → https://www.fastmail.com/help/clients/macmail-manual.html
Then this is just a convention for encoding Mac OS X or iOS notes into IMAP folders. I use this myself, and I can read my Notes in aerc fine, they're just normal mails inside theNotesfolder.
This further suggests that yes, they are just using the Apple Notes-in-IMAP encoding convention https://fastmail.blog/2015/12/24/note-to-self-imap-notes-are-go/