The best funny notes to write in Christmas cards include humorous commentaries on Santa, Jesus, Santa's elves, family, or the modern world. It's important to identify the specific audience for your intended card, such as a girlfriend or wife, and to cater the humor to that person.
One humorous comment to write is that the trait Santa and Jesus both share is that they both look good in a beard. Another is to talk about the government frequently raising the minimum age for drawing Social Security, which explains why Santa just won't retire. A sweet-but-silly message is to tell a friend the elves must be working overtime to make him a present because he has been very sweet this year.
Some family-related jokes include noting that Christmas only comes once per year and that around Thanksgiving, everyone becomes very grateful for this fact. A similar joking message is to note that spending all of Christmas season with seldom-seen family provides an excuse to neglect them the rest of the year. One modern joke is to ask if the Christmas tree is truly green ? as in, environmentally friendly. Another is to note that the chosen Christmas message was found on the Internet because the writer didn't care enough to personally write something.