So you have a character who's supposed to be a bit part.
But she becomes popular.
So popular that she crosses over into the comics.
And then people start fucking around with every facet of her character.
And that's what went wrong. The way they write Harley (and design her) as if you were writing Peter Parker as a muscleheaded brute and made him do his super-heroics in skinny jeans, thick glasses, and frosted tips