I don't really see where it matter show they're billed for casual conversations like this. I'd say Harley and Deadshot got pretty close to even screen time and that makes Margot Robbie just as much lead in my mind as Will Smith.
Same goes for Ben Affleck and Henry Cavill in BvS, they both get pretty much equal screen time and attention.
That's still your opinion though, as a fan.
There are indeed franchises where supporting characters outshine the main lead with some fans, that's for sure.
But this thread is about the main lead of the franchise, as per the top-billed. The lead actor.
WB definitely see potential for Harley/Margot as a lead, hence why they've announced all these multiple Harley spinoffs for the Emo Hot Topic audience. There is money they will want to exploit there.