I would have to say that Willy is both a victim of circumstances and also to blame. In the world created by Arthur Miller it would seem that both apply.
Is Willy to blame for his situation? most definitely. He made the choices, often bad ones. Willy must take responsibility but like most people it is often easier to blame other people or circumstances for our own failings. I.e. The teacher doesn't like me. Or the boss has it in for me. My dog ate my homework.
Willy is also a victim of the changing times. When Willy started out, a salesman would travel an area and have personal contact in a relaxed and casual environment but the times began to change on him. Technology had its victims and Willy Loman was one of them.
Willy loses Biff when his son discovered Willy's indiscretions. Biff could never live up to Willy's expectations and once the cat was out of the bag, so to speak, Biff lost all respect for his father.
Like most human beings, Willy is complex. He has his ghosts and demons.