The line has been attributed to several French princesses going back into the 17th century, although without proof. I have even read where Chinese scholars claim it originated in a very old tale or fable from there. Rousseau's mention of it is the first demonstrable use but he leaves the 'great princess' unnamed. He started writing The Confessions in 1765, finished in 1769, although it was not published until 1782.
If Marie ever used the line it would have been in jest. It seems to have been quite out of character for her to have dismissed the troubles of the peasants so cavalierly or ignorantly. She seems to have been sympathetic to their hardships or at least aware of them, even if she was blind to what her hubby was (or rather, was not) doing about it.
She was not credited with uttering this line until the mid 19th century.