Linda might be viewed as what is commonly known as an "enabler." Enablers typically care about someone else very much. However, they allow the person to engage in destructive behavior rather than endure the anger or disappointment of the person needing intervention.
In Linda's case, she could have confronted Willy with the findings of the insurance agent about the car accidents (they weren't accidents at all) or her discovery of the length of hose in the basement. She does not.
I do think it is a bit unfair, though, to say she was "detrimental" to his dream. Ultimately, we are all responsible for our *own* realization of our hopes and goals.