George Wilson worked at a garage on the outskirts of the “Valley of Ashes,” so one could make the obvious connection that he was covered in ashes because he worked there. However, the symbolism and the irony of the ashes covering Wilson go beyond that.
The “Valley of Ashes” represents the wasteland that Fitzgerald sees between the wealth of people like the Buchanans, and the disparity and poverty of people like Wilson. It is a bitter irony that Wilson asks Tom Buchanon to sell him a car that Tom owns, so Wilson can try and make more money for his wife, who is having an affair with Tom.
Wilson wears his “ashes” as a constant reminder of what he represents, and in fact is symbolically “buried” in this “Valley of Ashes.”
Fitzgerald is a very visual writer, and, frequently, his visual descriptions are very rich and often serve as symbols or metaphors. Wilson's appearance is very visually obvious and unusual, which means readers are meant to note his appearance and consider its significance.
Ashes are frequently associated with death (as with the saying "Ashes to ashes, Dust to dust," which was traditionally spoken by gravesides before burial, or "Ashes, ashes, we all fall down," which refers to a rhyme common during the time of the great plague epidemic in Europe), as well as destruction. The image of burning and ashes also tends to symbolize an end to something: an era, a way of life. In this way, Wilson's appearance could be seen a foreshadowing of death.
Myrtle's death is probably what is being foreshadowed here, since she is Wilson's wife. The "fire" that takes place before the "ashes" appear could refer to the heat and passion of Myrtle's affair with Tom, which ends in calamity. But Gatsby's death also occurs at Wilson's hand, in retaliation for Myrtle's accidental demise. The death of Jay Gatsby also suggests the "death" of the time of decadence and extravagance. His lavish parties and exorbitant spending to maintain a way of life that could not be sustained were like a slow-burning fire, fueled by his passion to pursue Daisy. This passion could not be sustained in any reasonable way and thus ended in tragedy.