Yes, it is completely normal to wish your favorite fictional character a happy birthday, why do you ask???
This reminds me of when I was younger and wrote heart-adorned letters to my celebrity crush for his birthday and listened to all his songs all day long, ignoring all the "you're crazy" comments my family used to make ... whatever, it is officially Dean's birthday today, so many happy returns, bb! I'm just wondering: did they ever address the boy's birthdays on the show? I can't think of an episode in which they did ... like showing if they get each other presents or go out to have a drink? This needs to happen, seriously. I want Sam to sing Dean "Happy Birthday" and bake him a cake! :)
Anyway, have a good one, Dean sweetie, and don't let the Sammy-soul-business drag you down, alright? ;)