I read it as an ironic comment of the narrator:if Lady Lucas was more clever, Mrs Bennet wouldn't be interested in her because they couldn't communicate...Mrs Bennet is not able to appreciate someones wit, as it is obvious in the case of her own daughter, Lizzy.

"...though I must throw in a good word for my little Lizzy."

"I desire you will do no such thing. Lizzy is not a bit better than the others; and I am sure she is not half so handsome as Jane, nor half so good-humoured as Lydia. But you are always giving her the preference."(Ch 1)