Mr. Knightley says, "to have you now, in thoughtless spirits, and the pride of the moment, laugh at her, humble her -- and before her niece, too -- and before others, many of whom (certainly some,) would be entirely guided by your treatment of her."

Emma has not only made fun of Miss Bates, but she's done it in front of people that have not known them Emma's whole life, like Harriet and Frank, who might well treat Miss Bates with less respect than she deserves if Emma feels so free to do it.

People of wealth and position could afford to take some public ridicule. They had the built-in power of their station to sustain their reputation. Emma has emphasized the ridiculous side of Miss Bates in a very public fashion. It should be Miss Woodhouse's job, as a person of more social power, always to emphasize the good side.

Emma doesn't ever make fun of her father's ridiculous side. She would be shocked to the core by anyone who did. Yet she expected Jane Fairfax to accept that Emma feels free, publicly, to mock Jane's surrogate parent.