I wouldn't call him a dominant postseason goaltender, he's consistent, and consistently very good, but his postseason resume lacks a truly standout performance. Even if you credit him for the mediocre teams in front of him, he doesn't have any particularly shocking upsets (Washington this season and 2007 Atlanta are the closest, and those teams were a combined four points better than the Rangers), New York has crumbled under adversity after gaining the upper hand against great teams (Drury in 07, 3-1 series lead to the Caps in 09), and the one time he had a strong regular season team in front of him, they lost to a 6th seed, with Lundqvist spotting his opponents three and two goals in the last two games of the series (with New York battling back to tie each contest before surrendering the winner).
Lundqvist's playoff resume doesn't really add to his case for me.
And he wouldn't have to bomb those five, just play far enough below average to drive his S% down two points, and there's basically no way to calculate how much the extra work effects the rest of the games he's playing over that period of time. That season is the fewest games he's played since 2006, and also happens to be his highest save percentage. While I don't think it's a complete career outlier, I do think his numbers are slightly inflated for this reason.
And even if he gets a few more wins with that greater workload, Smith and Quick's superior save percentages and more difficult team situations (LA's terrible offense/Smith's absurd amount of shots faced) probably get played up a little more.