I don't believe there is any way to do this in the core. You could add a relation between the User entity and the Moderator entity, and then use $user->Moderator->is_super_moderator, but this will incur an additional query unless you can eager-load the Moderator relation everywhere you need it.
It's worth noting that is_super_admin works the same way (it will incur an additional query unless the Admin relation is eager-loaded), though it uses a getter that first checks the is_admin column, which will mitigate this somewhat.