The Great Gatsby mocks the American Dream. We know that Fitzgerald himself was disillusioned with the times and his life, and in his frustration wrote of the failings of marriage without love and despair of those who had everything but nothing.
F. Scott Fitzgerald writes of a materialistic society, filled with greed and destruction, a society turning its back on all that is good but unable to escape guilt of sin as "God knows what you've been doing, everything you've been doing. You may fool me, but you can't fool God!"
--F. Scott Fitzgerald, The Great Gatsby, Ch. 8
I think that you can find evidence that displays how Fitzgerald believes the manner in which American ideals are pursued is akin to building a monument upon a firmament of sand. He writes about the social setting of the 1920s, the Jazz Age, with vivid detail. His depiction of the "flapper" lifestyle, the trappings of wealth, and the preoccupation with surface based notions of reality are all examples of the lifestyles he presents in the novel. I think his argument about this particular lifestyle is that it is not conducive to lasting values and something that can be sustained over time. It is for this reason that we see Nick walk away from this life and the people who stay in it are either doomed to being unable to enjoy life or are the types of people who will experience extreme challenges when the end of the Jazz Age is brought about by the Great Depression and the hard times that ensues.
To me, this question depends on what you consider to be American ideals. If the American dream is to get rich, then he is mocking it. If the American dream is to improve yourself and to be a good person, I think that he is embracing them.
I think you can see Fitzgerald mocking materialism throughout the book. The fact that Gatsby ruins himself trying to get rich shows that Fitzgerald does not approve of the idea of getting material goods without having some sort of moral foundation.
But look at who Fitzgerald uses as the narrator. Nick is the only truly decent person in the book. He is from the Midwest (metaphor for common-sense, traditional values) and he does not get corrupted by the East. By portraying him as the best person in the book, Fitzgerald is extolling his kind of values.