In her own comics she's still a side character, she's just a side character to Poison Ivy instead of Joker, and because its a woman controlling her its empowered I guess. Harley is a needy masochist. She works as an echo/enhancement to someone else who is strong, competent and manipulating. All of her successful comics follow that general theme. Anyone who comes in with the idea of her being a "feminist" and "independent" character already has a fundamental misunderstanding of what is appealing with the character.
I mean, its literally in her name..Harley Quinn...Harlequin, her name is based on what she should be, a servant. And usually a major fuck up. In fact one of her defining qualities is she is athletic and smart but usually fucks up Jokers plans anyway, which is why it looks like a twisted form of love, because Joker always takes her back and she gets attention from him that fuels her masochism and he also gets to continue playing with Batman (Because Harley fucked up his kill). In a twisted way, everyone gets what they want.
But its being a perennial fuck up despite being gifted that is one of her qualities that makes her endearing (She is a reverse Forest Gump) but also ensures she is always garbage on her own. She needs a competent person to be in control so she can play off them. And Joker, despite being a maniac, is super humanely competent, (I mean, he is geneuinly Batman's mirror, in fact he's MORE competent in numerous ways, he matches Batman despite being no match for him physically), as is Poison Ivy. Harley Quinn being on her own and not being someone's bitch is a huge mistake, she doesn't work well that way. She needs that competence to be a foil to and produce the fucked up interactions her character shines a light on.