I'm not an expert on the comics, but her first ever origin story was in BTAS, and that didn't involve her using sex to climb up the latter, right? Or did the same person who created her in BTAS develop her “dishonest-means-to-success” story? If it’s the same person, than I guess her dishonest means of succeeding in school would count as canon. But if different people developed it, couldn’t one say it’s up to interpretation? Personally, I don’t like that interpretation of her origin. (1/3)

As you said, even if she cheated in school, Arkham Asylum would have had to make sure she met all their qualifications- she would’ve needed references from professionals, she would’ve needed to pass tests. Someone would’ve immediately noticed if she had no idea what psychiatry was about. The reason Harleen spiraled into the Joker’s chaotic world isn’t because she was dumb. It’s because he is a master manipulator and has the tendency to psychologically damage everyone. (2/3)

Harley is still one smart lady. She may act silly, immature and childish, but that’s the point; Harley Quinn is Harleen Quinzel’s alter ego. It doesn’t mean she isn’t smart. Heck, the Joker himself is silly, immature and childish, and people still think he’s a genius. Plus, no idiot that has ever encountered the Joker has lived to tell the tale, but Harley has. I think that definitely says something about her intelligence. She is no idiot. (3/3)

Paul Dini (the writer who created Harley) wrote Mad Love, which tells the story of Harley’s origins and has a scene that depicts Harley having a sexual encounter with a professor. Mad Love, like the television episode of the same name, takes place in the same universe as Batman: The Animated Series and The New Batman Adventures, and therefore the events in the comic would be considered canon in the DCAU. 

I think a lot of people believe that Harley is unintelligent because she was manipulated by Joker, but IMO all that means to me is that they’ve most likely been fortunate enough to never encounter an emotionally abusive liar who preys on your insecurities and warps your perception through ploys for sympathy. Falling victim to a master manipulator can happen to anyone, regardless of how intelligent and educated you may be, especially if you’re in love with them and believe that they love you in return. 

People also like to forget that Harley managed to deceive and capture the World’s Greatest Detective using only her own prowess and without any help, and that Batman himself admitted she came closer to killing him than Joker–arguably his greatest foe–ever did.