Suicide Squad: 8 Reasons Jared Leto's Joker Is The Worst Part Of The Film
2. It Gives Him An Origin
Suicide Squad doesn't provide a definitive origin for the Joker, but in having Harley Quinn's transformation serve as a sort of ritual of insanity, we're inadvertently provided with a look at how he wound up the way he is; he too must have been doused in that chemical goo (which begs the question of why he didn't go even paler on the second dip). Beyond that though, it's a backstory he's willing to engage.
Getting an origin for the Joker isn't a movie-breaking thing - that's the first half of Tim Burton's Batman, after all - but it does weaken the character given how he's defined in popular culture. Jared Leto's Joker doesn't feel like this unknown force, but like a bit of crazed loon who could be a little dangerous if he had some back up. And that's not what you want from the Joker.
Like many of the changes made to the status quo in Batman V Superman - old, murderous Bats, zany Lex Luthor, dead Jimmy Olsen - this is something that's going to carry over into multiple movies (if the whole DCEU isn't massively course corrected), so will likely become more problematic as we go.