I have always thought about stuck in the middle as a song to/about his family, especially his parents or someone else he looks up to. It's about growing up. To me the title of the song is about not being a child anymore, but not yet really an adult, just stuck somewhere in between. And to start questioning everything you've learned and taken for granted, all the family traditions "are five kids better than one?".
I don't really think that this is an angry song, or something like that, I think it is about love, but all the pain and problems when it has to change it's form. An adult dosent love Hes/shes parents in the same way that a little child does, but he/she still loves them (usually)
I dont know if I'm making any sense or if you understand what i am trying to say. And it's probably all wrong anyway, I have no idea of what mika really thought of when he wrote this song, but this is what I think of when I hear it, 'cause it fits me.