The best way to determine who is the main character of a literary work is to determine how the plot changes the character, and whether the plot tends to focalize the point of view of a particular character.
For example, in Pride and Prejudice both Darcy and Elizabeth could be considered main characters. After all, the plot equally affects both characters and, in the end, they both change as a result of it. However, the novel is narrated through the point of view of Elizabeth; we do not learn a whole lot about Darcy's inner feelings, nor could we infer his emotions, unless he tells us himself.
Similarly in The Glass Menagerie, Tom is the character that is most deeply affected by the main issue, which is the stagnation of a family that is unwilling or unable to change. His status as a main character has nothing to do with the fact that he is the narrator; even in third person omniscient narrations it is easy to spot who is the dynamic character.
In this case, although Laura is who owns the glass menagerie, it is obvious that she will not change because her social anxiety prevents her. It is also obvious that Amanda will not move away from Laura, nor will she move away from the trauma of having been abandoned and left a single mother.
Yet, Tom recognizes the tragic flaw of his mother, his sister, Jim's and even his own.
[Jim's speed] had definitely slowed. Six years after he left high school he was holding a job that wasn't much better than mine.
Accepting a tragic flaw is what eventually leads the character to change and become dynamic while the others remain static.
Therefore, since Amanda, Laura, and perhaps Jim are unable to change and move forward (for whatever reason), whoever is most affected by the plot, to the point of changing altogether, is considered the main character.