Yes. You can customize the users that are created in your MariaDB or MySQL database service by including the endpoints key in the service definition.
When the user configuration is customized in this way, users will be assigned a password instead of the default blank password, which you can retrieve by querying the PLATFORM_RELATIONSHIPS environment variable during runtime. Please note that the password itself can't be customized.
For more information on this feature, please see the following page in the Platform.sh documentation:
https://docs.platform.sh/add-services/mysql.html#multiple-databases