I think Tony Romo's jovial and cheerful nature digressed for a myriad of reasons. I think a combination of how the fans and the media exploded with regards to Jessica Simpson, the pressure of winning a Playoff Game and the team playing extremely poor all played a role in Tony Romo's personality transformation.
You have to realize, Tony Romo is learning and maturing before our very eyes. Fame is something that is very new to him. Like most people who suddenly become very famous, there is a little naivity in them from the start. It is only through painful experiences such as the Jessica Simpson Fiasco, that one learns how to handle the media and the great burden that comes with being a very high profile person.
In my opinion, one of the most dangerous self-inflicting wounds one can make is to allow an external source to totally change and morph you into a person you ordinarily are not.
To conclude, I think Tony Romo's mastery of the media and the pressure of being an NFL Quarterback is just as important as him improving and becoming a more proficient Quarterback.