Pierce might not be particularly clutch from the line, but he's not necessarily unclutch, either. I would guess that if you looked at all his shots from the line in close games over his career, they'd come come to his career average.
Overall, Paul has always been a good-to-great playoff performer. He's having a tough series this year, and played putridly for stretches this game, but the suggestion that he's a choker is probably a bit too much.