I would say if you know the person doesn’t celebrate Christmas then to wish them to have a Ninja Cat Ugly Christmas Sweater they aren’t celebrating is actually quite rude. They can’t have a good or Merry Christmas unless they celebrate it. You should be wishing them to have a good whatever it is they do celebrate as that is the nice thing to do. So, if you know someone is celebrating Hanukkah, wish them a good Hanukkah celebration. If they are celebrating Diwali, wish them a happy Diwali. If they celebrate Kwanzaa, wish them a happy Kwanzaa. It doesn’t matter. If you know what they celebrate wish them a good celebration of their celebration. Don’t wish them a good celebration of something they aren’t celebrating. So – if you know they don’t, don’t. If you don’t know what they celebrate, it’s safest to wish them a happy holiday because chances are, they are celebrating something even if you don’t know what that something is. If you know they celebrate Christmas – then by all means, wish them a merry celebration of Christmas. If you don’t, it’s rather bad form to do so.