Are the MariaDB and MySql drivers interchangeable when connecting to a MySQL backend Aurora cluster?
Can MariaDB connector (driver) be used instead of the MySQL connector (driver) when connecting to AWS Aurora cluster MySQL instance?


Yes. I've personally used neither the MariaDB Connector/J nor the MySQL Connector/J against AWS Aurora cluster MySQL. However, according to the MariaDB documentation on its Connector/J - my emphasis:


  MariaDB Connector/J is used to connect applications developed in Java
  to MariaDB and MySQL databases using the standard JDBC API. The
  library is LGPL licensed.


So, MariaDB's Connector/J officially supports MySQL databases, and Amazon Aurora MySQL is supposed to be a drop-in replacement for MySQL. 

Also, note that according to the same documentation, MariaDB Connector/J has a special 'aurora' failover parameter. See MariaDB's Knowledge Base article Failover and High availability with MariaDB Connector/J for more details. 

I've not found anything specifically mentioning Aurora in the MySQL Connector/J documentation, so it would seem MariaDB's connector could actually be a better choice.  

It should also be mentioned that the licences are different: MariaDB Connector/J is LGPL v2.1+ and MySQL Community Connector/J is GLPv2 with a few additions.