I pretty much dislike everything about Harley Quinn, from her back story, to her aesthetic, to how she went from a bit player villain to front and center in the DC universe. She does nothing for me.
And even I recognize how huge she is. DC isn't shoehorning her into everything to "make her work." They're just giving people what they want. They said she was the fourth pillar during the Rebirth announcement. I'd argue that she's actually their second pillar now, right behind Batman.
Any success that Suicide Squad has should be credited to her popularity and morbid curiosity about Leto Joker. Nobody outside of comic book fans knows a single other character on that team. Harley will put butts in seats. It's up to Warner Bros to do the rest.