Social Darwinism was formed by people not as smart as I, because I realize it is not that easy--possibly even impossible--for mankind to anticipate which genes endow someone with superior survival traits.
For example, I can see a good argument that any genetic tendency toward a high IQ is possibly as inferior as one toward a low IQ.
I wouldn't measure the success of mental functioning by IQ at all.
I'd measure it by how well a person's brain works in concert with that person's gonads--at least as far as to what was most fit in prior generations.
As for fitness in future generations--that MIGHT be another story.