There’s some weird assumptions being made here–for example, a cremated body would not end up as a 13kg pile of carbon, almost all of it would be lost–but most of your basic facts are correct. The number of humans is actually 200 (there were 177 cars).
If you did somehow extract all ~13kg of carbon from each of the 200 passengers pictured, you’d end up with 2,590,000 grams of pure carbon dust. If you then formed that into a single diamond, you’d get an object quite a lot smaller than a car. I couldn’t find a way to calculate the size of a diamond from a known mass (apart from doing a bunch of algebra, and I didn’t want to), so I used https://www.omnicalculator.com/other/diamond-carat and put in some ballpark numbers for diameter and depth until I got close to the target mass.
I ended up at a sphere of diamond about 128cm in diameter. Still a big fuckin diamond, but you could put a bunch of those into one car, and it would be a lot smaller than the satirical pile of cremains in the meme.
Ok, so each person is a little less than 20% carbon, so a pile of 177 bodies would contain about 2000 kilos of carbon.
Dont forget that almost all of that carbon will be lost as CO2 during oxidation.
