Joplin wouldn't start, reinstalled it, lost all my notes

If you're using the Snap, uninstalling snaps deletes their config files closer to a mobile app than a traditional desktop one.

Snap keeps around a backup of the state of the app for every upgrade to allow reverting an upgrade alongside the databases, and also keeps a snapshot on removal.

If so, you might be able to identify the snapshot associated with the uninstall, and restore it.