I think that the ruptured hopes that Jim represents is indicative of the real world. Amanda and Laura view Jim as the "gentleman caller" who will be the magic solution to their problems. He has dreams and has aspirations and makes more money than Tom. Yet, over the course of the evening, it becomes really clear that these hopes become extinguished. Jim's marriage to someone else, the fact that he really does not understand the full extent of Laura's state of being, and that he really cannot fulfill the expectations that daughter and mother have placed upon him becomes evident. In this light, Jim represents how the real world does not quite materialize one's dreams. In the end, the subjective seems to not be mirrored in the objective. We see this with Tom and Amanda, and to a lesser extent, Laura. There is some level of disconnect between the internal and the external. This is the same with the expectations and hopes placed upon Jim.
This is a good question that can be answered from different perspectives. One, Jim is representative of the real world because he was the only visitor that the family had in years. What this means is that Jim precisely represents "the" outside world from which Amanda and Laura hide, and the world which Tom tries to escape.
Also, Jim represents what "once was" and could not sustain. A former school hero, most likely to succeed, impressed everyone, and had the admiration of Laura in secret. Yet, none of those promising things came to anything, and this is representative of how bad the economy, and the world around them was going. Life was not to be rosy after all, for any of them.