Deployment Manager does not currently have any special behaviour for mirrored databases.
If I understand mirroring correctly then one server instance serves the database to clients (the principal server) and the other instance acts as a hot or warm standby server (the mirror server). Therefore, I think you should still be able to set-up DM to deploy to the principle database server. The standby database server will update using the standard SQL Server synchronization (outside of DM).
So, you'd just need to create a database server target in DM for the principle db server.
If you do find you need to provide some special behaviour around deployments to your mirrored server - such as validating that the principle server is available - then we'd recommend writing a pre-deploy powershell to perform those actions, in combination with deployment variables
. Examples of what Powershell can be used to do can be found on our powershell forum here
I hope that helps.