Fuck, just watched this and it's indeed a mess. First half hour, Harley Quinn is completely useless at everything, then suddenly she becomes an all-powerful ass-kicking machine. Now clearly if she's the hero she has to be able to kick ass but Robbie isn't terribly convincing - she's no Charlize Theron, for instance.
Also, the villain is crap, and while the film is about recovering a diamond with some kind of secret to it, there's zero effort expended in making us care about any of this or showing how powerful or dangerous the diamond is. In the Maltese Falcon it's a stupid bird but you understand why people kill for it. Not here.
It makes heavy use of voiceover and flashback, which aren't in themselves bad things even though lots of people say they are, but the filmmakers seem too embarrassed to have worked out how to do it properly. The likes of Zombieland, Deadpool, Bridget Jones use VO, fourth-wall-breaking, and stuff, to build up a relationship with the audience, let us in on things, establish a bond, but in Birds of Prey or whatever it's called, the voiceovers are so dull and functional and tell us stuff we don't need to know. Men wronged her! Wow, that's surprising news. And why does it use non-linear time? Because that's edgy.
It doesn't help either that Margot Robbie's voice is fucking annoying and the film can't really decide if she's morally reprehensible, a loveable rogue, or a hero. As I said, is she clueless or bad-ass? Harley Quinn is a character who (to be kind) different writers have different interpretations of, but make your mind up, choose a side. The other Birds of Prey are potentially interesting but all horribly underwritten too. Doing a good superhero movie is hard, requiring a command of tone to avoid complete ridiculousness, it needs skill orchestrating action sequences, and keeping a human element. HQ and the Sorceror's Stone doesn't do any of that: there are some quite good bits, but it doesn't work at all as a whole.
And why the fuck does she wear a t-shirt with her own name on it? The sort of thing only the worst bands do. I'm not even sure it's better than Suicide Squad, which was awful in many ways but at least had a plot and some characterisation.