It is considered more personal to write a birthday wish inside of a card because it is a personalized message that was intended only for the recipient. While cards can often have humorous or sweet messages, adding a message shows that the sender of the card is sincere.
An example of a small message might be, "I am so thankful that you are a part of my life and I hope that you have the most wonderful birthday." A longer birthday wish might include mentioning memories that the sender and the recipient have shared together. In either case, make sure that the card that is chosen represents the spirit of the birthday person.