You're probably alluding to Marie Antoinette. While her infamous quote "let them eat cake" seems to be falsely attributed to her and many stories about her see

You're probably alluding to Marie Antoinette. While her infamous quote "let them eat cake" seems to be falsely attributed to her and many stories about her seem to have been fabrications, she did contribute to the downfall of the French monarchy through her high personal expenses and hardline stance against reforms. I'm not sure her role in the collapse of the monarchy can be compared to Elon Musk's role in tanking Tesla though - views of the monarchy were and would have been unfavorable without her already and her behavior wasn't much of a deviation from before, just made more significant because of the poor financial state of France at the time.