Honestly, I never liked Harley Quinn as a character, because it never really made sense. Why would a psychotic murderer have this doofy sidekick/girlfriend around? It doesn't really make any sense, and actually seems incredibly ghastly if you think about it. Like, who wants to actually envision a serial mass murderer with a love interest? Does the idea of Jeffrey Dahmer with a romantic partner make your stomach turn? It does to me. I don't think the Joker would have any interest in fucking. That seems like it'd be the last thing on his mind. And secondly... I don't want to think about the Joker doing any fucking. That's just, no... uh, nope, do not want. Ughhhhh. So that was a facet that I never bought. And it's certainly unthinkable with Heath Ledger's Joker. You can't picture a universe where his Joker has a Harley Quinn by his side. And that sat just fine with me.
In BTAS, there's a high level of devotion to the Joker. But why is she so devoted to him? It's because she's somewhat in love with this animal and believes he loves her, right? But does that actually make sense when this is a madman who, as stated in DKR, has probably killed hundreds and maybe thousands of people? Does that sort of dichotomy really work? I mean, it works when you've got this light and happy Joker in BTAS who throws pies in people's faces and punches people with extendable boxing gloves and, as far as I can tell, never kills a single person. But it kinda falls apart when you're trying to have her paired with the mass murderer Joker. It doesn't make sense that Harley would actually be in love with this "psychopathic, mass murdering, schizophrenic clown with zero empathy." Harley herself has always been portrayed as this goofy light-hearted ditz. Her airy, carefree ways don't jive with an insane murderer in any way shape or form. Could you see her leaning over the Joker's shoulder, smiling cheerfully as he shoots Barbara Gordon in the spine? I can't make it work in my head.
And let's talk about the Joker in the animated series. When I think of the Joker, I think of very specific and brutal instances. I think of him shooting Barbara Gordon in the spine, I think of him beating Jason Todd with a crowbar, I think of him twisting his own neck in laughter, I think of him buzzing someone's hand to death, I think of him slamming a pencil into someone's head in TDK. These are powerful images that resonate in the collective consciousness of fans. I don't think the Joker in BTAS really contributed to that at all. I cannot think of one Joker moment in BTAS that can live with those other insane instances. BTAS was many things, and it's a fine addition to the Batman canon, but its Joker failed on some level for me.
Ultimately, I treasure BTAS for its poignancy rather then its darkness, which isn't all that dark. When I think back to my favorite moments of BTAS, they're mostly things like... the ending to Heart of Ice, or the ending to House and Garden, or the ending to Baby-Doll, or the ending to Feat of Clay, or the ending to Mad as a Hatter, or Beware of the Gray Ghost. Those are all poignant moments, not dark moments. That's where the series succeeded for me.