She was great lol, honestly I thought she was hilarious. This is a pretty good example of why blood line limits, arranged marriages, and "keeping it inside the family" is the next best thing to giving into the slow death of over-eating. You never know who's going to pop out and decide that all she wants to do is give birth and spend money, at the same time.
Marie Antoinette was a child (12) when she had to marry the King of France. Additionally she was never actually educated to know the etiquette of high society in Paris' monarchy...
So I think she was just completely incapable of doing any better.
Irresponsible. Selfish. In my opinion, not a very good leader or ruler. However, I think decapitation was a little "extreme" for what she had done.