Should you say Merry Christmas if you do not celebrate the holiday?

Answer: Yes! Why not?
It is so much fun to say, and it just goes with the positive holiday feeling.
I said it to a friend who only celebrate the Chinese New Year, and although he laughed, he did respond, and it did feel great to just say.

No reason to keep yourself out of the loop.
Merry Christmas is fine- no mater what religion you practice, it is just about the feeling. The positive feeling.