Mobs from spawners don't count towards mcmmo, only naturally spawning mobs count. So only Dark Room Mob Spawners would work, and they are only so efficient.
From my experience, large platform-based enderman farmers are quite effective, myself having reached rates of an enderman every 5 seconds when lag is low. It could potentially be efficient in gaining MCMMO, and I can test that, if necessary.
Yes, Enderman farms do give mcmmo skill. However, they are usually designed to weaken them by falling, which is the worst you can do, mcmmo wise. So you only a very small amount of mcmmo skill, less than when you just run around and kill mobs.