Hi,
MongoDB is not tied to one cloud platform or any other specific usage model. It can be installed on-premise, in a cloud, or used as a database-as-a-service in all three major public clouds (Microsoft Azure, AWS, Google Cloud Platform).
Azure already offers a service called "Azure Database for MariaDB", so yes MariaDB can be hosted on Azure.
For the assessment and migrations, have a look at the Azure Database Migration Guide over here:
https://datamigration.microsoft.com/