This has been discussed ad naseum but very high and low IQ is detrimental according to research. Not that you'd need a study to know that ugly scrawny nerds and down syndrome looking retards aren't popular with women. Of course chad wins either way but there arent many genius or sub 80 IQ chads.
As for the correlation between looks and IQ? I'm not convinced and it probably has to do with positive reinforcement giving a boost. Better quality of life, more socialization, better treatment from teachers, less stress and mental illness makes for healthier brain development/higher IQ.
I was by far the top performer in my school til depression fried my brain. Highest test scores, never studied, way ahead of my peers as a child. Teachers wanted me to skip a grade or two..... And I'm ugly.