Of course not. Look, I'm as liberal as they come in America, and I think that's ridiculous. Christmas is a widely celebrated holiday in the USA. It's not like we're forcing it upon people who don't want it. I'm accepting of people celebrating Muslim and Jewish holidays. So I would expect that it be okay that I can say Merry Christmas any time I want!
Besides, it's never hurt anyone before, has it?
Also much of the Christmas holiday isn't exactly religious motivated (in America anyway). In many countries you have religious teachings on Christmas and very different traditions. Here we give gifts, eat Christmas dinner, have Santa. None of this is very religious related anyways