Historic footnote: That particular McDonalds had been called to the carpet multiple times prior to the incident of the parbroiled labia. They knew two things: 1) They were cranking the temperature up on their coffee machine to dangerous levels, 2) The Health Department in the area ordered them to reduce the temperature to safe levels.
Problem was, that McDonalds was getting their cheap, crappy coffee elsewhere and they knew that it was cheap and crappy. They had an economic need to keep people from tasting it until they were far down the road, hence the disfiguringly high temperature.
So, was the woman stupid for handling her cup in her car? Yes. Was that McDonalds liable for her injuries due to the unacceptably high temperature of said coffee? Yes. Had the temp been within the bounds the Health Department had set, it would have been a painful lesson, but not disfiguringly so.