I don't know that we can say it is capitalism or a capitalist system that leads to Willy Loman's failure and his personal tragedy.
True, he fails to make a success of himself. But there are other characters who are successful and happy. This would seem to suggest that the fault is not in capitalism, but in Willy.
Willy likes to play upon tropes of commercial success because they present a convenient, easily understood value system in which esteem and respectability are highly visible/systematicly defined. Loman plays on these notions of American success only because he cannot achieve them. They are an escape. They are a fantasy.
Maybe we can look at capitalism in this play that way: Capitalism is arbitrary and fantastic, while humanity is struggling to find its own way into acceptance.
This view doesn't mean that capitalism is the culprit though. It just means that the system is separate from the human drama (and only interposes in personal life according to the strength and weakness of humans).