As others have noted, the writings of Clark Ashton Smith were not in fact included in Gygax's Appendix N. Indeed, as I recall, Gary never read a word of Smith until Rob Kuntz suggested he do so and this was after the publication of OD&D (Someone like Allan Grohe can correct me if I'm wrong about this). I personally find this odd, because, in my case, I discovered CAS at about the same time I discovered H.P. Lovecraft (1981 -- the year both Castle Amber and the first edition of Call of Cthulhu were released) and the two authors have always exerted equal amounts of influence over my adventures and campaigns.
Yet, for whatever reason, Smith seems to be the least well known of the Big Three of Weird Tales. It can't be because his writings weren't widely available. Arkham House, August Derleth's publishing house, which he founded in 1939 specifically to bring HPL's writings to a wider audience, produced a Smith anthology in 1942, before even a second volume of Lovecraft. Entitled Out of Space and Time, its contents were personally selected by Smith as his best and included some of his most famous short stories, most of which are set in his signature settings of Averoigne, Hyperborea, Poseidonis, and Zothique.
Averoigne and Zothique are by far and away my favorite of Smith's settings. Both have a decaying, decadent air to them that I find strangely attractive in their repulsiveness. Far moreso than Lovecraft, whose writings simply state that the history of the Earth is long, far longer than mere men can comprehend, Smith's writings allow us to feel that longevity. The result is not despair at mankind's insignificance in the cosmic scheme so much as a crushing sense of intellectual boredom, an overpowering ennui that reminds us that there is nothing new under the sun -- it's all been done before and probably better.
These are feelings I have in my darker moments and that certainly explains my fondness for Smith and the influence he's had over my worlds of the imagination. I often wonder what D&D might have been like had Gygax been more familiar with Smith's writings than he was. Along with the better known, Robert E. Howard and H.P. Lovecraft, I feel each of these authors offers a unique but complementary perspective on the broad genre of "pulp fantasy." There's plenty of both Howard and Lovecraft in D&D, but barely any Smith at all -- a pity.