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.
You probably meant to reply to the parent comment, as this is one of the "weird assumptions" I explicitly called out (that almost all of the carbon would be lost).