Sumomaxxing has a lot of logic behind it.
I'm not obese but I'm a bit fat while still being athletic and I like it that way. Eating is enjoyable, which is good for your happiness (and thus your general mood/attitude) providing you don't become obese. Fat increases your practical strength (harder to push back, easier to push others back, more leverage to crush on top) and durability (more padding and mass makes you better able to absorb punishment/impact). This correspondingly increases your physical confidence, as well as your physical presence (intimidation) in a positive feedback loop. And having more fat means your muscles have to work harder to carry it, benefitting you in terms of strength a little bit. Weight gain also won't purely be fat if you eat nutritious food and a lot of protein: some will inevitably turn into muscle or go to maintaining it, especially if you are physically active.
Assuming societal collapse, fat is a long-term energy reserve as well as an insulator, so skinnycels would be the first to die.