What'd you say, they were "pretending to be grand, mature statements?" How so? Like you were quick to point out, there are plenty of moments in TDK that require your disbelief to be suspended - like the Joker's timing, plans, "adolescent" themes (don't think that makes sense), but do you honestly think Nolan was unaware of any of this? Very doubtful, it's just that he recognized he was making a BATMAN MOVIE lol. For that mere fact, it's clear Nolan wasn't intending to make this any grand statement, or an overly-pretentious drama film - just a good Batman movie, which he absolutely did.
Anyways, I just thought it was funny that your whole post was a reply to someone saying "Nolan's films had adult IDEAS." to which you disagreed. Even if you don't think the movie's realistic, or you perceive it as an attempt at being "overly serious" just because that's what critics said; disputing the fact that it had "adult ideas" is a pointless endeavor. A dude gets hanged in front of the Mayor's window for crying out loud - that's adult enough, we don't even have to touch on the themes or nature of the film. That scene is enough to solidify the "adult ideas" statement in my opinion haha.