In addition to the film's idealistic, feel-good ending, its premise is also unrealistic and romantic. If you keep in mind that Idealists and Romanticists trust wholly in the power of the individual, you certainly see that concept stressed in Mr. Smith. One man, Mr. Smith, is able to enter into the lions' den (so to speak) and come out unchanged and victorious. In contrast, if you compare the film to what happens to most politicians who spend time in Washington, DC, you will see a stark difference between the film's portrayal of how much of a difference one man can make and what generally happens to greenhorn Congressmen and Senators in real life.
Similarly, Jimmy Stewart's character is simply too good. While it is refreshing for viewers to see a noble, innocent politician take on the corruption of Washington, DC, it is more realistic to realize that humans are imperfect and that even those with the strongest moral codes often break down under intense pressure.