That's funny because I saw the opposite. Although I'm not convinced that Mercy is playing anyone of significance in the play that hardly matters. Arya is using traits that Sansa had/has to achieve success. That is to me the opposite of criticism, it shows appreciation, perhaps an aquired or taught apreciation, but neverthless. There may be a cynical undertone in the writing but it may just as well be about others not seing trough the act, not seeing things for what they are. Maybe Arya suspects that part of Sansa's behaviour was an act, too.