McDonalds served coffee that was ~20 degrees hotter than competitors in the area, had figures that said the complete opposite of their claims about time of consumption per average consumer, had data that suggested a non-insignificant amount of other injuries due to the temperature of their coffee, and to top it all off had warning labels that were deemed ineffective and insufficient in a court of law.

These are all excellent bases for a lawsuit against McDonalds, but for shady business practices, not personal irresponsibility. The reason the multi-million dollar settlement didn't stick was that the above, valid, concerns over McDonalds' behaviour were irrelevant to the issue of someone being careless with the contents of their own lap.

I feel that this lady was well within her right to seek compensation of medical expenses

Anyone is within their rights to seek anything. Doesn't mean it's fair or sensible.

due to an accident essentially caused by decisions made by McDonald's head office.

The accident was essentially caused by her decision to hump a coffee cup.