B here is violating Grice's maxims. That's the point. He's not following the cooperative principle. He's trying to insult A (perhaps because he is frustrated with the conversation). So applying Gricean reasoning to deduce B's intended meaning is incorrect.
If A asks "why are you changing the subject?", B's answer would likely be something along the lines of "And your mother's face is ugly too!".
B here is violating Grice's maxims. That's the point. He's not following the cooperative principle.
Then he doesn't get to complain when people mis-get his point.
Another month has passed and here is a new rationality quotes thread. The usual rules are: