To be a sin, it would have to violate either 1) Thou shalt love thy God or 2) Thou shalt love thy neighbor.
Now I do not accuse you of being a plinker for having curiosity about this issue, but to me this sort of question can be for some what I call "theological tiddlywinks," if it is part of a concern about issues which have nothing to do with rules 1 & 2. What the Lord really wants of us is our love and for us to love each other.
Santa Claus, I believe, is derived from an historical character (maybe the same as Saint Nicholas) who gave gifts at Christmas, a noble practice, to be compared with the gift-giving in Revelation 11 done by the wicked in their cause.
Yet, I did not emphasize Santa Claus in my home with my children, and I told them from the start that it was a just a tale, this modern idea. Since it is the day we use to celebrate Christ's birthday, the focus should be on him. And I never liked the British counterpart, "Father Christmas," since God is our Father. But neither to I charge those in Britain with violating rules 1 & 2 with this, as that would be judging.