I'm not very well-versed in the Iron Man comic books, so I couldn't say if the portrayal of Tony Stark was "accurate" or not, but I don't feel that the lighter, funnier side of Tony Stark conflicts with his darker side, his weaker side that leads him to become an alcoholic.
Many people with substance abuse problems are cool people to be around, who seem genuinely fun and carefree. It's only later, when you get to know them better, that cracks in the facade will become apparent and you get glimpses of the darker soul that has a serious problem. They'll hide whatever troubles them inside from the rest of the world and lie to themselves to great extent.
So I think Favreau was clever and indeed faithful to portray Stark as happy-go-lucky in this film and save the darker stuff for later. It's called building a character arc...