It depends how you want to perform backups. Do you want to backup all the members of the DAG or an individual mailbox server? For example I have 3 members in my DAG, 1 in my DR and 2 in my active site. When I do a backup of my databases I do it in all 3 of them, at the same time and I do the verification later in the day at the DR mailbox server.
If you only want to backup 1 mailbox server then you just have to install SME in that server. The other members of the DAG will not be backed up which may be fine you just won't be able to use the gapless DAG backup copy functionality and also you will not be truncating your transacation logs at the mailbox server not running SME. Are you using cicular logging if not how do you plan on truncating the log files on the members of the DAG that aren't getting backed up?
Are you going to be using other NetApp functionalities such as SnapVault / SnapMirror between your active/passive sites?