I maintain that this movie could [have] work[ed] if they leant into the musical angle and theme, compose original stories and couple it with insane imagery and a mind-bending story.
But by all accounts this movie is a courtroom drama and worse of all.....
a jukebox musical
Who is that meant to appeal to?
Why would you go down such a different and unique direction for the genre and not commit to it!
By not having Arthur fully embrace the Joker identity you undermine the ending of the last film, by having a sequel you are upsetting the fans of the first film who felt there was no need for a continuation, by having a courtroom drama and not a film of him committing crimes as the Clown Prince then you are disappointing the expectations of fans who wanted a sequel, by being a musical you are already departing from the usual fans of the genre and by being a jukebox musical you get zero points for creativity and are not making yourself appealing to the critics or musical fans.......
its just a series of easily avoidable bad decisions.
Batman doesn't need to be super cereal or grounded to work, hell I would argue that has been the problem with Batman in recent years, you can and should still have heightened elements and yes I believe a Batman musical could work so long as the producers are not ashamed of it being a musical, commit to the idea and write and compose original music for it.
Here are a couple of great Batman inspired original songs from Miracle of Sound on Youtube that show how well this concept could work:
Also this song from the German stage musical Tanz Der Vampire/"Dance of the Vampires"[which itself is based on Polanski's Fearless Vampire Killers] was actually originally written for a cut musical sequence from Schumacher's Batman Forever
So I maintain the idea has merit, if done by someone with a passion for the project and experience with musicals and decently composed original songs.