Sullen is to moody as monotonous is to what?

Monotonous is to dull, boring, tedious, droning or repetitious  Monotonous means the lack in variety.  

Musically speaking, it means an unvarying tone or pitch. But since your analogy is started by an adjective, we have to pair it with an adjective too.

So you can pick one from any of these: dull, boring, tedious, droning or repetitious