Yes, the word order suggests you really are mean. That is not the same meaning as you are really mean! Word order sometimes is very significant in English!
i agree, but it's close enough in meaning that they should accept both answers. If they're only going to accept one, I think it should be that "You really are mean." But that gets marked wrong. :-(
With the current word order, doesn't it stress more on the verb? You really are mean! If not, how would you say that?
The word la usually suggests that the next word will be a noun? So...maybe: "You are a real meanie!"