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.