Though I do not agree that Dirk chokes (as bad as people say), the numbers you posted are extremely misleading.
Nowitzki did not make the playoffs the first two years he was in the league, aka the two least statistically productive years. If you take those years off, he is 24.06/9.3 and 47.4%
Also, his playoff numbers are weighted heavily towards his most productive years, where he played the most playoff series (leads his team to success in his best years, always a good sign).
So you have to look at his numbers on a year to year basis for comparison.
Year Reg PPG RPG FG% PO PPG RPG FG%
2001 21.8 9.2 47.4 23.4 8.1 42.3
2002 23.4 9.9 47.7 28.4 13.1 44.5
2003 25.1 9.9 46.3 25.3 11.5 47.9
2004 21.8 8.7 46.2 26.6 11.8 45.0
2005 26.1 9.7 45.9 23.7 10.1 40.2
2006 26.6 9.0 48.0 27.0 11.7 46.8
2007 24.6 8.9 50.2 19.7 11.3 38.3
By looking at the numbers this way, we can tell that, in 3 out of the 7 years, he had a significant increase in PPG, scored about the same in 2 other years, and had a significant decrease in 2 years. His rebouding numbers have consistently increased quite a bit in the payoffs, while his FG% has been generally lower in the playoffs (not surprising, I think most players get lower FG% in the playoffs).
In conclusion, at least statistically, it doesn't seem like Dirk is particularly clutch or non-clutch.