Actually, there are studies out there that make a very convincing argument for people in 2015 being born smarter than those in 1915. And those born in 2115 will be smarter still.
The details are pretty vague, so I could be remembering this incorrectly, but if memory serves the basic premise of the study boils down to social evolution, cultural preferences, and educational opportunities all combining to create a human race that is incrementally more intelligent than its forebears.
We might complain about the failing quality of our schools, but it wasnt that long ago that most of the world's population couldnt read or do basic algebra. More education means more intelligent people, as learning has been proven to increase IQ via "brain exercise". That higher IQ is passed down to our kids, who themselves enjoy a higher degree of education than we did (most likely, anyway), which increases their IQ's, and so on.
Culturally, we have little need for big strong men to protect our women and children, and more need for intelligent men and women who can nab good jobs and provide a more wealthy, stable life style. We're breeding raw, brute strength out of the human race in favor of intellect, and that's been going on since the discovery of agriculture. With the advent of modern industry its accelerated quite a bit.
So, going by this, I would imagine a planet like Krypton, well ahead of us in its development, would indeed be a world of genius-level people.