Lizzy could have simply responded by praising Charlotte and moving on. She didn't have to denigrate Mr. Collins.

What if the shoe were on the other foot? What if Mr. Darcy had said something like, "My aunt really is an arrogant, officious old bat. Could you believe what she said the other day, about how, if she had ever learnt to play the piano, she should have been a great proficient? What a piece of work!"

It seems to me that Lizzy would have been made uncomfortable by this intimacy, and would have thought that he shouldn't be insulting his aunt to her, a near-stranger, despite the obvious truth to what he said.