She was a joke, and we liked the joke at first. "Oooo, the Joker's so craaazy, trying to get inside his head will turn you craaaazy too, oooooo!".
Also, she wanted to fuck him and he wanted to fucking kill Batman, a man turning sex down is always a haha funny joke moment because a man would never do that irl amirite lads?
Over time we got sick of her stupid voice, shit design, and boring personality. She's either another henchman but with tits, or a "Big cool girl" trying to live life without the Joker, getting all the revenge moments Barbara wishes she could get.
Fuck Harley Quinn, she's a shit meme character whose presence makes the Joker unable to be funny, because no writer these days has the balls to even slightly portray domestic abuse as anything other than the evilllest worstest thing ever. No jokes, no funny cool Joker riding a christmas tree into the sunset, only edgy asshole joker slapping and then quiet-talking his girl into staying like every other boring abusive asshole boyfriend character ever.