To be fair, we've never seen her cry either in Her form. I guess when you been exposed to all the evil in the world, it tends to leave you emotionally blank. Mimi probablly has anhedonia by now. Or maybe (given that Raven was her teacher) she has developed Raven's reluctance to show deep emotion for fear of losing control.
"For I have heard the Song of War, and to me, it sounds just like "Please, Mister Custer.""
Cogito Ergot Sum, "I think there is something wrong with this rye bread." (Aplogies to Seafrol.)