*cough* My opinion is that, no, The Dark Knight isn't abnormally popular because of Heath Ledgers death. That is silly.
. . . its abnormally popular because it was a high quality DC movie in a time when DC spent a decade failing to produce movies while Marvel was putting out multiple films a year. Then in comes The Dark Knight, Jesus to BB's John the Baptist, to show why DC "really is" the superior comic line after all, thus heralding an era when truth and righteousness once more rules, and where everyone forever after acknowledges the supremacy of DC over Marvel.
( And then the Avengers came out, completely changed the pop culture landscape, and demonstrated that the new benchmark of eternal victory was, in fact, just an outlier spike in quality. And some people have still not forgiven Marvel for that. )