The best part about evaluating "clutchness" strictly by the stats, is that LeBron's uncontested layup with 9 ticks left last night counts just as much in most/many "clutch rankings" as Durant's big time 3 against the Lakers in Game 4.
Defining a shot where the shooter drives right at Kevin Garnett and ball fakes in the air to get him out of the way as "uncontested" is a bit amusing, but I agree with your greater point. Even the "5:00 left in the game" and "neither team ahead by more than 5" designations are arbitrary.A relatively inconsequential shot like the one you're talking about - cutting a lead from four to two with 10 seconds left while the defense is more interested in preventing open three and avoiding fouls than giving up a two - shouldn't carry the same "clutch" value as a shot that ties the game, takes the lead, or even a shot like Pierce's late three that put BOS two possessions up with less than two full possessions left on the clock.
ETA: at the risk of sounding too patronizing, questions/critiques like the one you just registered is exactly the type of thinking the led to the discovery of advanced stats like eFG% and O/D Rating.
It's how the next pass at attempting to measure "clutchness" will be better than current efforts to quantify it.