I think they are largely uncorrelated, in fact I'm willing to go out on a limb and say that they are anti-correlated. James Maxwell, who everyone regards as absolute genius, was a famously bad teacher. My own grad experience also adds that an absolute beast of a researcher is not necessarily the best teacher. I believe the term "curse of knowledge" is popular enough that it suggests people who are way too deep into their abstractions seldom rise above it when it comes to communication. Hence the stereotype of nerds being socially awkward and hard to communicate with.