If "clutchness" actually exists, at least in the sense that people usually mean it -- ie, someone who actually gets _better_ than they normally are in clutch and/or playoff situations -- then it ought to be measurable and consistent, and it isn't. This is easy to demonstrate in a stats-heavy sport like baseball, where the example I like to use is that Mr Clutch Derek Jeter has hit better than his overall career numbers in the ALDS and World Series, but considerably WORSE than his career numbers in the ALCS. The guys who hit better than usual in two-out RBI situations one year don't do it year after year. Etc.
You can't demonstrate any of this with stats w/r/t NFL quarterbacks in the playoffs, where all they get judged on is whether their team wins the game or not, and stats are considered to be beside the point. But the rush to explain everything in terms of character-driven personality traits is even worse. For example, for years everyone blamed the Colts' inability to win in the playoffs on Peyton Manning being a choker, rather than his team's inability to put together a defense. Matt Ryan's about to be in the same boat. Eli's teams had a great defense, so now he's got a reputation for "ice water in his veins." Most of it is silliness.
There may be some hope, though, in that the expected "Joe Flacco Is An Assassin Who Knows How To Get It Done On The Biggest Stage!" groundswell hasn't happened. For once, everybody seems to understand that that's nonsense.