[Kanji_The_Wanderer]

There is no evidence in the story that Misa is mentally unstable at all. Her parents deaths making her turn Goth is purely opinion. Her fashion and living habits were never explained in the story, and it is up to the reader to decide for themselves. Sure it may seem that a lot of Goth people have problems in their life and they like dark things. But saying this about everyone who dresses that way is a stereotype. Maybe she just likes to wear black lace dresses and fishnets, and is interested in strange things (like weird dolls). The only thing that I would say is "mentally unstable" about Misa is her infatuation and obsession with Kira for saving her life and avenging her parent's deaths by killing their murderer.