It's your fault if you run into the back of someone no matter if you are overweight or not. Most of us know of people being rear ended with cars that are way underloaded.
In addition, many vehicles of all types are overloaded on trips. Most of us have noticed obviously overloaded vehicles including SUVs and sedans. Sometimes, it's simply because they carry too heavy of people or luggage.
I suppose it's a valid argument to say the wreck cause was because a vehicle was overloaded, but I don't think it will factor in fault. Even underloaded, it's still your fault if you run over someone.
I think the more reasonable argument is to not drive over the capability of your vehicle. Still, sometimes wrecks will happen.
There are multiple causes and effects while driving. Here's a question ... Does the person who never drives over 62 but is overweight have a bigger or smaller safety margin than the person who routinely drives much faster but is underweight? Obviously, there are other factors. Weight is just one.
That being said, I chose my F450 for better brakes, tires, and my combination is not overweight. :)
