I am not a dev, just a user, but I have had a look into how Joplin handles note data.
The answer to your question would be no, unless you have taken steps yourself such as using something like a Veracrypt volume or full disk encryption.
Locally your notes are stored in plain text in your ~/.config/joplin-desktop/database.sqlite file. Your resources / attachments are stored unencrypted in your ~/.config/joplin-desktop/resources folder.
As an aside, if you are thinking of requesting local encryption and / or a passcode lock (or similar), please have a search of this forum first as the pros & cons have been discussed many times and it is also somewhat covered in the FAQ.