In our U.S. History Shmoop learning guide on the 1920s, we talk about the huge rise in consumer culture at this stage of the twentieth century. Well, "Winter Dreams" was published in 1922, right at the start of the Roaring Twenties. Fitzgerald is clearly responding to the sudden, visible signs that lots of Americans are getting very rich, very quickly. Dexter Green's ease with making money demonstrates both the positives (yay! cash!) and the negatives (boo! bad human relationships and loss of romance!) of this sudden rise in American wealth. The obvious and visible class differences between the very rich and the middle class are what drive Dexter to succeed at any cost, even if it means losing the romantic idealism of his boyhood.
Dexter gets that being part of the upper class is about more than having money: after all, he insists on attending an elite college and dressing perfectly for his first dinner date with Judy.
Mortimer and Judy Jones' disrespectful behavior towards the people is Fitzgerald's way of telling us that wealth kills your sympathy for others.