In "On Social Class and Happiness," does Margaret Cavendish support or criticize her society's status quo? How can I write about the rhetorical strategies she used, and what would be the thesis if I were writing essay on it?

You were pleased in your last Letter to tell me, that you had been in the Country, and that you did almost Envy the Peasants for living so Merrily

From this line, it can be inferred that the prevailing attitude in Cavendish's society is that the wealthy live happier lives than the lower classes. Note that her friend, to whom she pens these reflections, is not actually envious of the poor. She's "almost" envious, hanging on to the idea that she's better off with her wealth and isn't going to trade that in any time soon. Cavendish wishes to further investigate this idea.
She thinks that the lower classes really do live happier lives than the wealthy. They don't have as many possessions to cause them anxiety. They get to be outdoors and enjoy nature, not worrying about how the sun will damage their complexions. They attend dances and other social gatherings with friendship and fun, not with an envious attitude borne in comparisons. She contrasts this sharply with images of opulent balls:

But great Ladies at Publick Meetings take not such true Pleasures, for their Envy at each others Beauty and Bravery Disturbs their Pastimes, and Obstructs their Mirth, they rather grow Peevish and Froward through Envy, than Loving and Kind through Society . . .

Cavendish does seem to have a slightly romanticized view of the life of the poor in her society. After all, are the poor excited to fall into bed in utter exhaustion each night so that they don't have to worry about sleeping well? Are they that happy to have to earn their food by the sweat of their brow? Probably not.
Cavendish uses both pathos and logos in her reasoning. Pathos (an appeal to the emotions) is seen in all the images where the poor seem to be living a joyous, carefree, exuberant life that the wealthy can't experience. They milk cows instead of curling their hair and dance without restraint instead of worrying what people think. They are free to live life for themselves, on their own terms, instead of living simply to impress other people. Logos (an appeal to logic) is seen in the section where Cavendish comments that for every happy noble, there are 100 happy peasants. She also closes in logic, noting that it is impossible to know the thoughts of anyone and that outward appearances can be deceiving.
If you are looking for a thesis that brings all of these elements together, you may want to consider something like the following: Through pathos and logos, Margaret Cavendish challenges the common belief in her society that the wealthiest people are the happiest people.
The body paragraphs could examine the overall assertion of how she sees the lower classes as living happier lives and then where, specifically, pathos and logos are used in that argument.