USB generally tops out at 4.5W, with most USB ports being less than half of that. A 12V power outlet in a car is generally 60W or more.
A 60W heater can provide just over 200 BTUs of heat: much, much less than any reasonable camping stove.
I might use a USB-powered thing to maintain a warm beverage, but I don’t think it would work very well heating cold water, and I sure as heck wouldn’t want to try it for boiling even a small amount of water.