They definitely should've explained what happened to Joker
Christopher Nolan said he didn't out of respect to Heath Ledger, which I think is ridiculous.
I don't believe in ghosts, but if I did, I'd say Heath Ledger would probably feel extremely patronised. I'm sure he wouldn't want his death to have a negative impact on the movie.
Joker was such a huge part of the story, having him suddenly disappear with no explanation feels very strange.
Christopher Nolan said everyone already knew why he wasn't in The Dark Knight Rises, which was probably true in 2012, but kids today might not know about Heath Ledger's death. Just imagine if they watched the entire trilogy in one day, and a very important character suddenly disappeared with no explanation. I think they'd feel very confused and let down.