What traits of Mary's are being mocked? why would austen mock a person having Mary's character?

Mary is a caricature of the bookish young teacher sort. Unfortunately,despite all her reading, Mary really doesn't seem to comprehend much. She is mocked as being less than intelligent. She lacks the sharp wit that Elizabeth naturally has. I think Austen would mock people she considered "posers". Austen didn't like people pretending to be something they were not.