Plain text?? A user's password is hashed...You don't need a paint text password to compare text encrypted with it to other text encrypted in the same method. Either they match or they don't. The user's hashed password doesn't leave the server and plain text is never involved.
If you use the password's hash as a key, the plain text no longer matters, because the hash is now the thing that an attacker needs to forge a credential. So your database is effectively storing the real password, as if you had not used a hash.
Even if you didn't you still need to retrieve the user and password from some storage to validate the key, which invalidates the reason for JWTs in the first place, since you supposed to be able to validate them without access to an auth service/db