Miller shows the problems of buying into the American Dream of capitalism, free enterprise, and competition without scrutiny. At the beginning of the play, Willy has been reduced to working on straight commission. After working his entire career as a salesman, he still must borrow money from Charley to make ends meet. The idea that the dream is available to all who work hard is exposed as a myth.
But Willy shows that there is more to the American Dream than just working hard. He also believes in luck, the kind of luck his brother Ben had, who walked into the jungle and when he came out, he was a rich man. It is this idea, the idea of seizing opportunity, that he passes on to his sons Biff and Happy. Their pipe dream of creating a sports line through a Happy/Biff basketball exhibition stems from this view of the American dream. Miller shows that some people can make it big this way--Ben is an example. But most are not that lucky or that ruthless.
And yet, some characters are able to succeed in the traditional sense. Bernard, like Biff, comes from humble roots. Bernard studies hard, goes to college, and becomes a prominent lawyer.
So, to anwer your question: Miller is not sympathetic toward unchecked capitalism. The pathos of the scene in which Willy talks with his boss Howard supports this idea. It is a cruel system that dispenses with employees when they are of no use. But neither does he wholly criticize it. Some, like Ben and Bernard, can make it in this system: either by luck and opportunity or by hard work.
Miller's depiction of Wily and his relationship to the world seems to be the ultimate statement against capitalism. In a world defined by the "American Dream" and its drive for success in a materialistic or socially deemed manner, Wily and his condition is something that is seen as the negative response to such a predicament. Essentially, Miller is asking two questions. The first would be that in a world where everyone is told to "succeed," what happens to those who don't? The second question is more jarring. If capitalism is predicated upon those who will be successful and those who will not, can it be the fundamental base for a dream such as "the American Dream?" Is this even fair to subject people to impossible realities that will end up crushing them in the end in order to keep those in the position of power entrenched in their respective positions? When Miller suggests that the play is about the matrix that most people face in their lives, it seems to be a severe questioning of capitalism and the free market principles that have lulled Americans to sleep in presuming that this mode of existence is a perfectly morally/ ethical one.
In the play "The Death of a Salesman" Willie Lowman, a name chosen to indicate that the man feels low, just can never seem to get ahead. He bites into the American Dream that if he sticks with his job and works hard, he will be able to find success.
I do not think that Miller was very fond of what capitalism represents for the average man. Willie is a symbol of the average everyday man trying to own his own home, car, and raise his family. Willie doesn't ask for much but what little he works to achieve ends up with him falling further behind.
The person who seems to have success is the man whose father owns the company and he gets the company from his father. This is also indicative as to how the system has continued to provide through inheritance and class. Willie had no such advantages.