It is possible to make AppImage auto update and it would be cool to see the Joplin AppImage have this feature.
AppImage apps with auto updates
Here's my workaround:
alias updatejoplin='wget -O - https://raw.githubusercontent.com/laurent22/joplin/master/Joplin_install_and_update.sh | bash'
I get notifications from GitHub when there's a new release, so I just run updatejoplin in my terminal.
But I agree, that would be nice.