Mmm...am not sure if Mimi is undead or not. Maybe what she means by being already dead that she died once but some how she came back to life, & she knows she suppose to be dead yet she still lives. Either way things are getting interesting.
I wonder if she means it literally when she says she's dead, or if it's just that she's been reduced to pulp so often that her mind has started to break. Kinda like how Sayaka did in Puella Magi Madoka Magica. "I'm just a husk".