In the context of the play "Romeo and Juliet", Friar Lawrence says this to Romeo in Act III, scene iii after he is crying like a "girl" (according to the Friar) about his banishment after killing Tybalt. Friar Lawrence is trying to tell Romeo that he is behaving like a cry baby and that he should be thankful that he was not killed and only banished.
The quote "Like a misbehaved, sullen wench" would be a comparison of the character to someone who is sulking, who likes to act up, who is often in trouble. Someone who is sullen is sulky, sort of angry and ticked off. Therefore, someone who was behaving "Like a misbehaved and sullen wench" is being a troublesome, sulky, pain in the butt. :-)