Is cereal popular in other countries?

It's popular in some countries, certainly over here in the UK. You're right about the marketing shizz. In other places, it's regarded as being quite unhealthy and garbage. Junk food.

In Asian countries, many people eat little pancakes stuffed with grilled chicken, pork, onions, rice and other tasty stuff for breakfast. Japanese folks eat "onigiri", which is basically a ball of sticky rice with something quality in the middle. It's sometimes wrapped up in seaweed, which you can also eat if you're feeling particularly peckish.

Porridge, oats and corn are still used as a way of filling your stomach in many parts of the world.

do you just mean 'breakfast cereals' - or do you mean cereals in general, the plants on which most of the world live. Rice, wheat, rye, barley, oats, maize [what you call corn] - these cereals are absolutely essential in every single country in the world.

Kellogg's Corn Flakes were originally marketed to reduce the sex drive in teen aged boys in the early 1900's

Depends on the country. In developing countries - probably not so much, at least in comparison with the more developed ones.