WB has overestimated the popularity of these characters in general.
Harley Quinn is popping but in relationship to her being a Joker side character, not a solo character, and certainly not a team leader of a bunch of c list villains no one cares about.
marvel did the same thing with mystique. without magneto she's a mid character too.
use a character that has some popular solo comic runs. if you want them to be a solo character. Jessica Jones for example had a widely lauded alias comic, and netflix based a jessica jones show (season 1 only) about that comicbook run. and unsurprisingly is was well received