I'm also running 2012 R2 as a Domain Controller and nothing unusual to report.
Whilst Avast is compatible with
Windows Server (ie, a member server) I've never seen anything to do with having to set exclusions for Active Directory database for a Domain Controller for example.
IMHO exclusions are just for performance reasons, so on a small single-site DC I'd doubt it matters much. But I certainly would be checking the Microsoft documentation for recommended exclusions for individual products like Exchange and SQL Server (which are added on in SBS too) as they can create huge files and would suffer performance. I know Microsoft have KB documents like that floating in their web somewhere, or might be in the product install guides. It doesn't hurt to put the exclusions for another AV product into Avast if you are concerned