Oh Birds of Prey... There is a lot I can say about this movie but I am going to try to be polite with what I feel was wrong with the movie. Don't worry, I'll keep the politics and costume design out since it has been done to death on both Twitter and YouTube.
1. The marketing and development. It is clear that WB wanted to recreate Marvel's success with Deadpool. I can imagine what the meeting was like:
WB EX 1: "Deadpool made Marvel a lot of money and is the highest grossing R rated movie of all time. What do we have that can give us the same success as Deadpool?
WB Ex 2: "Well we have Harley Quinn. She is DC's best selling comic book character next to Batman. She is basically bubble gum Deadpool. Why don't we make a movie about her. With her popularity and sex appeal we'll make more then Deadpool!
WB Ex 1: "Brilliant! Get on that right away! And make sure that it is R rated too like Deadpool! We can prove to everyone that we too can make an edgy R rated super hero movie!"
WB Ex 3: "Sir, Todd Phillips is making a solo movie about the Joker who, by the way, is the most popular villain in comic book fiction and maybe all fiction in general. Are you sure that we shouldn't be putting our eggs in that basket instead?"
WB Ex 1: "Pfffttt... No way! Harley Quinn is a better character to get behind then the Joker. Besides, angry people on Twitter with blue check marks and pronouns in their bios told me that Joker is going to be incel baiting trash that is going to cause a mass shooting. Ew! Don't want to be anywhere near that. In fact, take most of WB's stock out of the Joker movie and put it into Harley Quinn instead. That is a much safer option to go with."
Oh the irony...
Yes, it is true that Warner Brothers took most of their stock out of Joker over Twitter outrage and put that stock in Birds of Prey instead which means that Warner got little money from the Joker's billion dollar success. What a STUPID decision! Then again, hindsight is twenty-twenty.
Anyway, back to the marketing and development mistakes. Harley, unlike Deadpool who is mostly popular with older boys and men, is primarily popular with young girls between the ages of 10-20. The type of girls that primarily shop at Hot Topic or know Harley from that DC's Super Hero Girls Netflix show. With young girls, their parents are going to take one look at the R rating and be like "Hell no!" and instead take their daughters to see Sonic instead. Women over the age of 16 are also not going to see a hard R movie. Here is a dirty little secret for you guys reading this. Most women do not like R or M rated movies and games. Most women generally don't like hyper violence in general. Why do you think most women prefer movies like Bridesmaids or games like The Sims over movies like Predator or games like Doom? Sure their is a sizable female audience with properties like Fallout and Mass Effect but even then females only make up maybe at most 20 to 30 percent of the fan base. Warner really should have made the movie PG-13. Tone down the violence and market the movie as a gal pal night around the town movie. Granted, they still wouldn't have beat Sonic but at least Warner would have made a profit. Instead they did so poorly that Gail Simone, the writer of the Birds of Prey comic and all around nasty and vile person, went on a 3 day Twitter rant over men not going to see the movie and contribute to her royalty check which is stated to be small due to the movie bombing. Haha!
2. The Birds of Prey are nothing like their comic book counterparts. Where the hell is Oracle, the creator of the Birds of Prey team? Why is Montoya a 55 year old women when in the comics she is in her 30's? Did they seriously cast a black women to play Black Canary because her super hero name had Black in it? Cassandra Cain is nothing like her comic book counter part where she is this badass assassin. But the biggest peeve is that Harley Quinn had nothing to do with or was she ever a part of the Birds of Prey team. If DC and Warner wanted to make a Harley Quinn paling around with the gals movie then why not adapt Gotham City Sirens instead? You can make a fun, Thelma and Louise style movie with that comic. You can have Harley paling around with Poison Ivy and Catwomen, having fun and causing mischief while she tries to move on from her relationship with the Joker. Hell, you can even have her develop a deep friendship with Ivy and hint that it will develop into a romance like it did in the comics. At least it would have been true to the source material!