I guess that gets into the question of how much Harley's voice and accent in general are an inflection and something she deliberately does and how much of it is just her naturally letting herself loose.Melissa Rauch may be the WORST voice I have ever heard for Harley Quinn ever. Like all of her deliveries of even simply lines like "phew" felt forced and cringy. Felt like they only got her just to have a name not because of her talent. I'm certainly glade Robbie proved there is at least one lady from The Big Bang Theory who could voice HQ because it sure wasn't Rauch.
I feel it works with Tara Strong and to a lesser degree Hynden Walch is because those two are also very talented VAs know how to do a lot of voices and inflections and can also put a lot of energy and charm into their performances. Though yeah those are also for series and even movies that require a lot more energy and don't have low key moments like this show has. Like Robbie the first one I've seen even from Arlee Sorkin where the normal regular Harley voice, not Dr. Harleen Quinzel can work in just having normal conversations without coming across as obnoxious or anything so major props in that regard alone. But you don't really need those kind of roles in say the Suicide Squad DTV movies or DC Super Hero Girls show where Harley's is more on the side so can be more on point without having to carry more normal moments.
Fone Bone's point though is that with the ACTRESSES who are playing Harley the accent feels like a facade and fake when it shouldn't feel that way when we are listening to the character. And though I feel with Walch and Strong it's fine, Robbie to me yeah will never be a definite Harley Quinn because her voice can just grate on your nerves but not in a good or amusing way either. It's fine in shorter doses when surrounded by other people which is why The Suicide Squad and even Suicide Squad it wasn't that noticeable, but then you had Harley Quinn; Birds Of Prey (not going to use the super long name for that movie) that gave her a lot of narration and solo dialogue which man does not sell you on the character at all whenever she gives these long bits of narration. Which in that movie is all the time and really takes you out of it. Robbie's really good at the psychiality and look of the character but Cuoco works much more on someone you want to just listen to play the character.
Because generally Harley's voice is different from Harleen Quinzel, and other adaptions have taken to the idea that her accent is more pronounced as Harley because she's not trying to appear professional or as someone other than who she really is.
Of course, accent or otherwise, I think Margot Robbie has a fantastic voice