I’ve used Adminer a few times for debugging things, but I don’t recommend running it on production systems longer than necessary.
Simply put the adminer.php file into ./data/web in your mailcow folder. You can then access it at https://mail.yourdomain.com/adminer.php.
The server name is simply mysql. Passwords can be found in your mailcow.conf file, either in DBPASS for the mailcow user or DBROOT for the database user root.
I guess phpMyAdmin could work as well, unless it has special requirements, e.g. for PHP extensions.