They matter in different ways. High grades are like the good college. It's instant attention. If your goal is to work on Wall Street, good grades are an advantage. Just depends on what you want to do.

Similarly, high IQ implies smart, but low IQ does not imply unsmart.

Grades matter only if you have no other experience. If you want to work on Wallstreet chances are you don't have much experience coming out of school. But after you get the right experience nobody cares about grades. The question is whether or not you can get the right experience in the time you would otherwise waste worrying about grades.