Joker is meant to be a force of nature in TDK. The film has many of the trappings of a crime epic, but it's still a superhero movie at the same time. Which means it's still a heightened reality where crazy stuff (although nothing overtly supernatural) can and does happen. If I can buy into the verisimilitude of a guy in Bat-suit waging a one-man war on crime, I can do the same for a guy in clown makeup manipulating people, taking over the mob, improvising his plans and getting places (and people) rigged with explosives. I still think the realism aspect of those films is often misread by some. It was never supposed to be about explaining every nook and cranny in detail and proving "this could actually happen". It was about grounding enough of it in a cinematic reality more akin to other action/crime films that the larger than life characters and events are juxtaposed against that and carry more weight. While Begins did go through a lot of effort to make Bruce's transformation into Batman seem very credible, I think this was more of a foundation for the rest of the story to stand on than anything. It set the tone for what kind of Batman world this was. I'm sure if the Nolans wanted, they could've done the entire movie from The Joker's perspective and have it read as scarily plausible. But him being this unknowable force of nature adds so much to the feeling that he's going to be near-impossible to stop for Batman and Gordon. I think they did a great job of keeping the grounded tone, but also not demystifying too much. I mean, we see Batman perched up on some ridiculously high skyscrapers in the films. We don't need to see the laborious process of him using his grapple hook repeatedly, or whatever, to get to those spots.