How much of it is the marketing itself, instead of the film not leaving enough for the marketing? I think it's the film's portrayal of these characters causing the issues. It takes these interesting characters (with potential marketing power) and strips them down to realistic, gritty versions of themselves. Aside from Harley, what's the difference between Canary, Huntress, and Renee Montoya? Two are badass women in leather and the other is a badass woman with a leather jacket. And Cassandra Cain is a little kid in an R-Rated film.
Kind of a far cry from Guardians and Suicide Squad. Guardians had a wise-cracking Solo type, a badass woman in leather (but in green), an unhinged muscular alien, and a talking raccoon and tree. Suicide Squad had Will Smith as an assassin with a white mask and scope eye-patch, a human crocodile hybrid, a witch, a Mexican tatted up gangster with flamethrower hands, an iconic crazy clown couple, a wise-cracking Australian, and the straight man to balance everything out. Marketing team for both films had a field day.
BoP takes a platinum blonde in fishnets and strips that away (Canary). A detective in a fedora and faceless mask and strips that away (Montoya). The silent, ninja-assassin adopted daughter of Batman and, you guessed it, strips that away (Cassandra Cain). Only Huntress resembles her comic counter-part, but she never visually stood out anyway. I'm not arguing that comic accuracy would've put butts in seats, but the film chooses to change everything iconic and marketable about these characters.
Imagine if Wonder Woman wore her 90's leather jacket look instead of her iconic armor? Or Captain Marvel wore her Air Force outfit instead? Yeah you gotta make sure the characters are good in the film, but you also have to give people a reason to show up to said film. From a marketing standpoint, this film suffers by going the Marvel Netflix route.