I think it's just confirmation bias that stems from the meme. I've had four cats in my life: an orange tabby tomcat, a black tomcat, a white and orange girl and a grey tabby girl. The orange tabby tomcat was the smartest by a mile out of all of them and the grey tabby girl was the dumbest by a 100 miles. Orange and white girl is the second dumbest and the black tomcat second smartest.

But people aren't gonna post their average or smart orange cats or their dumb non-orange cats on the internet because it doesn't fit into a specific meme/sub/community. So now it seems like there's heavy correlation, but there really isn't.