Well, you shouldn’t be using plugins at all; they’re a dead feature walking, almost completely unmaintained, and slated to be removed entirely before too long. It’s nearly as easy to create a jail and run pkg install mariadb105-server, which will leave you with a maintainable solution.
But with that said, the plugin appears to randomly generate a password, which it shows you when it finishes installing:
If you’ve closed that, it’s also available inside the plugin jail itself in the file /root/mysqlrootpassword.