The best way to put this is that the 1920s were a time when people stopped being idealistic and started being much more interested in hedonistic things -- in having fun and in getting ahead financially.
Before WWI, there was a Progressive movement that was, as you say, very idealistic. Progressives were trying to change the world -- perhaps trying to perfect it. A perfect example of this was their attempt to legislate morality through Prohibition.
After WWI, this idealism seemed to be gone, perhaps because of how terrible the war had been. You can argue that people no longer thought the world could be perfected so they just wanted to turn their attention to having fun.
Because of this change, the '20s were a time of fun and materialism. People were getting rich because of the stock market boom. They were enjoying themselves with the new movies, the plentiful cars, and the exciting new music. They were coming to have different values -- ones that emphasized having fun.
Overall, then, after being idealistic before WWI, Americans turned towards having fun and getting rich in the 1920s.