I think it does to an extent. Some players definitely are stat freaks, and some seem to play better in high pressure situations, like Flacco.
It is all relative and when you start to have these discussions people latch on to one certain play or one game from a player and use that small sample size to make their point.
Take your example of Flacco... if the DEN safety/CB doesn't blow the coverage on that 70yd TD, the entire story of Flacco/Manning from last year is flipped.
It is reasons like this that I don't like to characterize Tony Romo or Flacco or whomever as clutch or not clutch.