I see what you're saying but i guess what bothered me is that if Lewis symbolized a lion for Christ, then why not symbolize Father Christmas too? It just strikes me as a bit unbalanced in that regard.
I see your point, but I wouldn't dwell on it too much. But if you need an answer, I'll offer this: what sort of other character or creature _that you should find believable_ would be on hand when the protagonists need them and provide the gifts they need to face the challenges ahead? Father Christmas/Santa is the only character that fits the bill. There are no "allegorical" Pevensies or street lamp (introduced in another Narnia volume) in Narnia, so an out is provided for Father Christmas to be there as well, I'm guessing.
Having Father Christmas also drives home an important point. What Jesus did on earth is to be universally celebrated in all of God’s Creation. The gifts of Father Christmas were not trivial; they were what were needed for to accomplish what was prophesized.
The Narnias must have had a knowledge of what Jesus (Asland) had done in the home lands of Adam and Eve. If I had to guess, Christmas was an integral part of Narnia since its creation (brought there by Queen Helen and King Frank in “The Magician’s Nephew”). Lewis also stressed that man made religions which existed prior to Christ coming were man’s attempt (although somewhat warped) to reach out to God and they had some truth in them. He incorporated this theme in both the “Chronicles of Narnia” and his space trilogy. He rejects religions which came after the Incarnation as see in “The Horse and His Boy” and “The Last Battle”. “Tash” is not “Aslan” and Allah is never to be confused with the Judo/Christian understanding of who God is.