When Biff discovers Willy's infidelity, he loses respect for his father and realizes that much of Willy's wisdom and advice is hot air, mere aspiration, or even a illusory fancy to which Willy clings as a clown might cling to his make-up.
The clown metaphor may be apt. Biff has discovered that Willy is, in some ways, a ridiculous fraud. Yet Willy is determined not to seem ridiculous. He never takes off his make-up in public - with very brief exceptions (such as his first talk with Charley).
In Biff's view, Willy has become a kind of actor. His act, however, has a noble origin. Biff can still ascribe hope to his father. Yet, Willy's hope cannot deliver him, or his family, into a situation of dreamed of material success.
For this reason, Willy remains a deluded and dejected figure, a failed tragic actor who cannot quite play his "character" successfully, falling short of achieving an ultimate meaning. This fits with Arthur Miller's own take on the tragedy of Willy's situation (and the tragedy of the many people like him).
"It is that we are struggling with forces that are far greater than we can handle, with no equipment to make anything mean anything."
Another effect of Biff's discovery is a shift in his affections. Where he once revered his father and sought his approval, Biff now sees that it is Linda who deserves respect (and pity) and who he grows closer to.
In this way, we might say that Willy has become a shadow of his formerly bright self (in Biff's regard) as well as becoming Linda's shadow.
Willy can also be connected to his car. He, like his car, never quite reaches his destination. As a man, he had aims. He had goals, and he fell short through weakness and distraction. Like his car, he breaks down on the way to the place he intended to reach. And perhaps also, like his car, it was the driver that failed. In the metaphor, Willy's dream of success is what drives him - and fails him.
[Willy] has misdirected his energies and talents chasing a dream that never had any chance of materializing.