To give a quick highlight, because this case is often politicized and misrepresented:

The plaintiff, Stella Liebeck (1912–2004), a 79-year-old woman, purchased hot coffee from a McDonald’s restaurant, accidentally spilled it in her lap, and suffered third-degree burns in her pelvic region. She was hospitalized for eight days while undergoing skin grafting, followed by two years of medical treatment. […]

Liebeck’s attorneys argued that, at 180–190 °F (82–88 °C), McDonald’s coffee was defective, and more likely to cause serious injury than coffee served at any other establishment.

So, the lawsuit never demanded McDonald’s to put a warning that you’re not supposed to spill hot coffee on yourself. It argued that it’s an unnecessary safety hazard, because the coffee was served at hazardous temperatures.
No matter how many warnings you put down, it can happen that someone spills coffee on themselves and they shouldn’t need to be hospitalized from that.