On one hand, you would think that evidence shows that the US economic system since WWII has been a very good system. The US continues to have the world's largest economy. It is a leader in many kinds of innovation and attracts immigrants from all over the world.
If, however, you must argue that it is not a good system, you should argue that it has not responded well to globalization. In the twenty years or so after WWII, the US did not have much competition from around the world. WWII had devastated countries such as Germany and Japan. China was not yet economically important in any way. During that time, the US economy (you can argue) grew to be complacent and inefficient. Since globalization has started and the US has had to face competition, its lead on the rest of the world has steadily declined. Today, it faces economic problems, particularly with creating good jobs for people without a great deal of education.
This struggle to adapt to globalization can be seen as evidence that the US style of capitalism is not a good system.