Given the problems with his sister, his mother, and himself, Tom's opinion that Jim is the most realistic character makes sense. Their hold on reality is so tenuous that in comparison any other human being would seem "realistic" indeed. He does have a normal job, normal aspirations. He has dreams that were unfulfilled, but he still pursues life. He is kind to Laura, and for that Tom is grateful, and he treats her as if she is a normal person. Perhaps there Jim shows his own lack of realism because in fact Laura is deeply troubled, partly because of her disability but more importantly by her immersion in her dream world and fear of everyday life.
At the beginning of the play, Tom, himself, tells the reader/audience that the play is written from his point of view. He has lived and dealt with Amanda and Laura his whole life and is writing their characters from his own perspective which is skewed with emotions brought forward by the memories he is writing. Tom does not have a plethora of emotions tied to memories for Jim; Jim is portrayed by his actions, not by how Tom viewed and remembered Jim's actions in relation to his own life.