Did Marie Antoinette really say "let them eat cake" because if not then why is it popular folklore?

No it is not accurate. Marie Antoinette however WAS known as "Madame defecit" for her careless spending habits. She was known to wear ridiculous hairdos bordering on insane. The folklore probably came from the common people's hate of the rich and was made known to support the idea that the nobility were selfish, worthless people.