Biff says about Willy that "he had the wrong dreams. All, all wrong." ("Requiem") Was Biff's assessment right?
On the whole, you'd have to say that Biff's assessment is correct. Notice, however, that Biff doesn't criticize his father for having dreams; it's that he had the wrong dreams. It's not that Willy was too ambitious or wanted something better; it's that he had unrealistic dreams that led him to make bad choices in life. Most people have dreams, and that's perfectly healthy and natural. But Willy's dreams are delusions because they're not related to who he really is as a person; they're not an accurate expression of his soul.
A good example of this is the brief affair he had with his secretary. Willy prides himself on being a family man and yet he was so corrupted by his unattainable dreams of success that he cheated on Linda with a woman who ultimately meant nothing to him. His dreams of being a hotshot salesman, a "well-liked man" led him to look upon having an affair with an attractive young woman as one of the trappings of success. And this, in turn, affects Biff. Once he uncovers the affair, the relationship with his father is ruined forever. Willy's dreams have impacted negatively on Biff, so it's not surprising that he regards them as being so terribly wrong.
Biff was correct in his assumption that Willy's dreams were "all wrong". Shortly before Willy's death, Biff began to experience an epiphany in which he realized the extent of Willy's fantasies and the effect they had on him. He noted that Willy lived a life of self-deception and false ideals, always chasing after a victory that Willy himself could not fully describe. Furthermore, Biff knew that his father's talent would have been better invested in chasing after the things that he really loved, which were nature, the outdoors, and building things.
In addition to this, Willy did not become a salesman because he wanted to become one. Instead, he did it because he once heard that a man named Dave Singleman had become successful as a salesman. Therefore, Willy was trying to repeat the success of another person, and did not try to go after his own.
This is also why Biff's assumption that Willy's dreams were all wrong is correct. Not only was Willy not going after what he loved, but he was also going after the dreams and successes of someone else.