I can see some validity in this idea. Joan represents much of Esther's own exploration of self. Joan's ability to remain detached from men and not capitulate to the social expectation of a woman having to be with a man is reflective of sentiments that Esther experiences.
Yet, I would have to suggest that the overall trajectory of Esther's characterization is that Joan is not a true mirror of Esther's sentiments. Esther's suicide and her own inability to survive in the world is something that Esther ends up rejecting. Esther is the survivor, the one who recognizes the challenges that exist in consciousness. Joan does not display this level of gravity.
When Esther is at Joan's funeral and remarks, "I wondered what I thought I was burying," it is an indication that she is not mirroring Joan's sensibilities. Esther has "buried" a part of herself, the part that felt the need to mirror Joan, in this moment of grieving. While Joan had no issue embracing her own sexual orientation as a potential path for clarification in the issues of consciousness, Esther does not do so. Her journey for understanding and the process by which she is "retreaded" is one that does not find the answers in sexual identity, as much as a psychological one. Finally, Joan's condition of wealth and Esther's lack of it represents another reason why Joan could not be seen as a true mirror of Esther. It is easy to move to this understanding given all the ambiguity that surrounds Esther. Yet, I think that her characterization is so rich and complex that it seems to avoid the idea of an easy mirror found in another.