Actually, in a way it is inspiring: he is exercising his own choice to live a life that he loves.
I don't mean that everyone with an IQ >120 should drop what they're doing an join OXFAM. The greatest help is usually done through research. Also, people aren't robots with infinite motivation. They have to be interested in their work to perform well. That said, he can probably find something he's interested in that also benefits others.
What's next? Millions upon millions of people have obtained undergraduate and graduate degrees in field X, and gone on to a career in field Y. Well, aren't they squandering all their hard-earned experience and knowledge in X? Were they duty-bound to pursue X instead? What's the difference?
I also think it's absolutely absurd to say someone's squandering their talents and not "aiding humanity" when they have a Ph.D. in physics, they're a civil engineer, and they've published ninety academic papers.
That's basically what I meant by "duty." Now to address the specific points in your post:
Nobody asks in advance to be born intelligent.
Correct. Currently life is very very unfair. Nobody asks to be born into poverty and ignorance either.
Can you think of any other trait one could be born with which you would say bestows someone with a duty to lead their life in a certain way?
These aren't genetic traits but they can be inherited: Wealth. Power, such as in a monarchy.
...aren't they squandering all their hard-earned experience and knowledge in X?
There are a couple of differences in your example. First, not all degrees are equally useful. A Ph.D. in Russian literature isn't going to be very useful in curing disease or engineering hardier crops. Second, education can't fully substitute for fluid intelligence.
I also think it's absolutely absurd to say someone's squandering their talents and not "aiding humanity" when they have a Ph.D. in physics, they're a civil engineer, and they've published ninety academic papers.
Compared to most people, yes, he has helped a lot. But if this guy really is the smartest person on the earth, he is wasting his potential. Bad things are happening -bad things will happen- that wouldn't have happened if he worked in biotech or physics or cognitive neuroscience.
Here's a silly but analogous situation: If Superman was a social worker, wouldn't you be frustrated that he wasn't doing superhero stuff? Sure, social worker Superman has helped more than most individuals, but he could do so much more.
The fact is that we all aren't born equal, but we all hurt; we get sick; we die. It's completely unfair. We need to fix it. The more great minds we have devoted to these problems, the sooner they'll be in the history books instead of the nightly news.
I really would not be frustrated that Superman was not doing superhero stuff. It's very hard to herd someone smart or capable into doing something they don't genuinely want to do, and I doubt it's usually productive in the end.
I have to admit, I take this personally, because I may not be Superman, but I sit around writing little stories that nobody but me will ever read, reading books that do little practical good but to fill me with a sense of beauty and contentment; I shoot some pool, play some chess, and I work on software that exists primarily to satisfy my curiousity or to make me money. Most of it doesn't help anyone else very much, and I like it that way. I think I have the right to do whatever I please, as long as I take care of myself and my friends without doing harm to other folks on the planet. When a man comes up to me, as has happened, and talks about how I have a duty to make an impact on the world, I want to kick that man.