Yes, Merry Christmas. Even if you aren’t currently a christian. Remember, however, that Christ is not just about one day in a year, He is every day in the year.
If you want to go on a guilt trip, though, read Alexander Hislop’s book “The Two Babylons”. Especially the part about Tamuz worship, yule logs and decorated trees during the winter solstice.
If you want even more of a guilt trip, read Jeremiah 10.
Before you go getting to feel too guilty, unless you are using the Christmas tree as idol worship, I should think you are ok.