Ackshully, the term ‘average’ usually refers to the arithmetic mean, but it can also signify the median, the mode, the geometric mean, and weighted means, among other things. It’s an ill-defined term, and I tend to use it that way to avoid pedantic discussions of statistical minutae in general interest conversations.
Back to the OP: I suspect that if there is any negative correlation between intelligence and success, it would be because highly intelligent kids don’t have to work very hard to succeed in school, and therefore never develop strong work ethics or study habits. I know that’s the case for me - I went through my school years without ever having to study. I could knock off a 1-hour math assignment in 10 minutes flat. So I almost never had homework to speak of.
I also got away with murder in terms of class attendance. It got so bad that by the time I got to high school I was skipping most of my classes. They let me get away with that because I maintained my grades anyway. Then in university I tried the same thing - and it showed. My marks in university were all over the map from perfect A+'s to F’s, depending on how much material I missed by skipping classes, and also by how much the class interested me. Interestingly, I got my best marks in the toughest courses, but average to lousy marks in the ‘easy’ ones which largely didn’t interest me.
It took me a decade after university to build up the skills I needed to survive in the working world. Now I’m quite successfully and have a nice home and family and all the rest, but I spent a LONG time flitting about from project to project, working on my own stuff, gambling for a living, etc. before I could settle down.