I definitely agree they could have easily made Harley Quinn a stronger character by at the very least, giving her shorts to wear instead of a pair of shiny panties, but overall I don’t think she was too terrible.
I interpreted that specific cringe worthy scene where her fantasy is the stay at home 50's mom differently. I don’t know if it was because it was subtle and a lot of people didn’t pick up on it or if I misunderstood something completely (also very possible), but I just didn’t see what you saw.
Earlier in the scene the gang is talking about how Diablo has to “own” the fact he killed his wife and children, they really kept repeating that. Then when the stupid Enchantress character gives everyone visions, along with Harley’s eye roll fantasy, was the vision Diablo had of his family alive and all of them hanging out together while he inappropriately smacked his wife’s ass in front of the kids. Then following Harley’s vision, if I recall correctly, I believe Deadshot yells ‘This isn’t what you want! She’s manipulating your mind!’ Not exactly those words but something to that effect.
If I remember that correctly then that completely changes the scene. Harley was being manipulated into thinking that generic stay-at-home fantasy was what she wanted by Enchantress who didn’t know her well enough to give her a viable fantasy. This adds up when you consider Diablo’s fantasy is also “not what he wants” because they just finished discussing the fact he needs to “own” his crime and that things can never return to those happy times, ending in his acceptance of having to move forward with life as a shitty person.
Those fantasies were planted into the characters minds by Enchantress as things she thought they would want after presumably peering into their minds to figure out who they value most. Harley is insane, there is no chance that’s what she would want, her realistic fantasy would probably be doing something insane alongside the Joker. None of those fantasies really added up, they were mind tricks. Just like the fantasy of Deadshot killing Batman, he shouldn’t really want that because he wouldn’t want his precious daughter to live in a world with no hero (he’s well aware he’s a villain).
With this interpretation of the scene, I don’t think Harley Quinn was too bad of a character. Her backstory tells us she’s insane and therefore not an admirable character just like all the others. She is super dependent on the Joker but that is because she is deemed insane. She is on the exact same ass kicking level as all the other anti-heroes (except Diablo who is clearly far superior in power than anyone else in the gang). She only flirts/insults in sexist ways because she is an insane psychiatrist- so presumably she knows how to push people’s buttons and how to interact in ways that get her what she wants. Sadly the best way to get testosterone fueled macho guys to do what you want is to flirt. There definitely should be a conversation when introducing this character to both children, but it shouldn’t be about this character and her perceived weakness- it should be why this strong, genius, yet insane character feels the need to manipulate and behave in such a way due to the sexist universe she inhabits.
With all that said, my biggest complaint against Harley is the lack of shorts and her constant silly one liners. They could have used her so well! But obviously the writers of this film were lazy as hell. The movie had insane plot holes! So many of them. Enchantress was the most sexist character in my opinion, she was meek and pathetic when she was just plain old Cara. The movie was overall misogynistic- the camera work alone proved that true. But I don’t think Harley Quinn was the movie’s problem.