Depends on the interpretation, in some she's really not that bright.

She's usually a licensed psychiatrist and very intelligent, although she rarely shows it in her Harley persona.

But then New 52 ****** her up and had her lie and cheat to get her diploma. Cause we can't have Batman villains actually doing anything that isn't bad.

In the animated series and the arkham games as well as some other comic series like those based on the animated series she's generally dimwitted

One can have an airheaded demeanor while still being intelligent. Especially when one is insane.

Harley's actually a very brilliant person! She is (i guess was) a licensed Psychiatrist and wanted to prove herself at Arkham Asylum, renowned as one of the craziest places on earth. Of course the joker was "Smarter" and manipulated her, but she is very clever.

She was a psychiatrist before she started ******* the Joker. But then again, in one of the comics it showed that she ****** her professor to get better grades.