One sleight of hand in Mankiw's column (as Paul Krugman points out) is his easy leap from the relatively unobjectionable way Downey earns his fortune to the way other executives — especially those in finance, who represent an enormous chunk of the very rich — earn theirs. If you don't like Downey's movies, you can choose not to attend them. If you don't like the financial industry siphoning off your 401(k) or plunging the world economy into a massive crisis destroying trillions of dollars and ruining the lives of millions of people forever, you don't have as much recourse. Mankiw wants us to ignore the serious moral problems embedded in the rentier class – such as the Wall Street titans joking in secret gatherings about their reliance on bailouts – and think of the one percent as Iron Men, to whom we owe admiration and gratitude. But even if we consider Mankiw's chosen example on its own terms, the Iron Man problem does not take us where Mankiw wants to go.
Mankiw is no rube. He is smart enough to know better, which is even worse.