This is a challenging question. My personal opinion is that Christmas is intended to represent the terrible racial problem that has existed in the Deep South since the inception of slavery, and especially since the end of the Civil War. Joe Christmas seems to embody the hostility that existed, and still exists, between blacks and whites in the South and also, to a limited extent, throughout America. Christmas carries that mutual antagonism between blacks and whites right within himself. The white half hates the black half of Christmas, and the black half hates the white half. He doesn't belong in either world.
Faulkner did not seem especially interested in “modern” man as such. If he were, he might have written about men and women living in modern cities rather than men and women living on the land and eking out subsistence livings as their forebears had done for thousands of years. Instead, he seems to have deliberately turned his back on "modern" man. He was more interested, it seems to me, in all of mankind. In his “Foreward” to The Faulkner Reader, he writes:
To uplift man’s heart; the same for all of us: for the ones who are trying to be artists, the ones who are trying to write simple entertainment, the ones who write to shock, and the ones who are simply escaping themselves and their own private anguishes. Some of us don’t know that this is what we are writing for. Some of us will know it and deny it, lest we be accused and self-convicted and condemned of sentimentality, which people nowadays for some reason are ashamed to be tainted with; some of us seem to have curious ideas of just where the heart is located, confusing it with older and baser glands and organs and activities. But we all write for this one purpose. This does not mean that we are trying to change man, improve him, though this is the hope--maybe even the intention--of some of us. On the contrary, in its last analysis, this hope and desire to uplift man’s heart is completely selfish, completely personal. He would lift up man’s heart for his own benefit because in that way he can say No to death.
Faulkner was influenced by Sherwood Anderson, mainly, it would seem, stylistically. Both men wanted to write in the American vernacular, using simple language to talk about simple people and to capture the American vernacular as it is used by simple Americans to communicate with each other. I don’t believe that Faulkner was interested in “grotesques” for the sake of grotesqueness—at least not in his best works. The characters he writes about can be strange and unique, but they never seem to be included to make the reader feel he is being introduced to freaks. Anderson’s grotesques seem to exist unobtrusively in a “normal” rural environment, whereas Faulkner’s might be said to be the creations of an abnormal rural environment.
William Faulkner was obviously a better writer and a deeper thinker than Sherwood Anderson. Faulkner has a growing international reputation, while Anderson is fading into oblivion.
But to be real honest, I would have to give your intriguing question a lot more thought.