In Linda's and the boys' eyes, she was definitely a support system for Willy. Like most women of the mid-20th century, Linda viewed herself as responsible not only to take care of her husband's home and physical needs, but also to prop up his ego, reassure his doubts, serve as a buffer between him and his children and live in his shadow. Linda did all of those things for Willy and she also suffered his verbal (and perhaps physical) abuse. She knew that Willy was self-destructing and felt herself helpless to stop it, but she stood by him to the bitter end. From Willy's perspective, he may have considered Linda detrimental to his dream; she always seemed to be the voice dragging him back from his retrospective wanderings with his brother Ben. But the play is not overt in this aspect and I doubt whether Miller intended for Linda to be perceived in this way. He seems to cast her in a sympathetic light; in fact, she seems to be the only character in the play who has a realistic and comprehensive understanding of what is really going on with her husband.
I have heard strong arguments for both sides, but I personally believe Linda Loman was detrimental to Willy's dream.
Linda allowed Willy to exist in a world of make-believe. She created a safety net of comforting support around him which allowed him to carry on a false existance.
I will even go as far to say I believe Linda is to blame for Willy's destruction. Linda pumped Willy full of lies concerning his work ethic, physical appearance, and social skills.
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.