... it can still be ambiguous in that it could employ an unreliable narrator to tell a story that has some truth to it, but has been skewed so much that you don't really know what actually happened.
I think it would work if we, as the audience, could see it played straight but with commentary from The Joker that doesn't fit what just happened. But really, there's a lot of ways to do it that could work.
The movie does look interesting, but the Joker is supposed to me a mystery. It feels like they're giving too much insight into how he became The Joker. We're not supposed to know.
Bleh... I donno, I still enjoy comic movies, but these offshoots have got a bit out of hand. Especially the villain ones. I mean, we're not supposed to root for the villain, but you kind of have to make the protagonist sympathetic or the audience won't care and then you end up with an anti-hero instead of a villain, kind of like with Suicide Squad or Venom.
And Harley Quinn, in some stories, actually does become an anti-hero and rejects the Joker. She became so popular, they made her a good guy. Venom, too, becomes an anti-hero later on, but The Joker is... well, THE JOKER!
Even if it's well done, you can't turn the Joker into Tony Soprano and it still be the Joker.