As I think about it, I agree with you. Though I didn't like the first Suicide Squad much, they probably did handle Harley better than the other films. Part of the problem may be that Harley is good in small doses, but if you overexpose her then the flaws become more and more evident. When she first showed up it was fairly novel and interesting (even though many of us were very familiar with her from the Animated Series), but the character doesn't have anywhere else to really go from there. I feel the same way about, say, Deadpool. A little goes a long way. But since she is essentially the DC equivalent of Deadpool they want to maximize her popularity by putting her in everything.
See I think he had a lot of potential to grow as a character and to be used in different ways in different films. He starts off as suicidal, has major issues with his mom, but starts to develop some confidence and friendship, then. . .gets killed. Unlike Harley. I guess it's character development that she goes from loving Joker to wanting to kill Joker, but she doesn't really learn anything or change over time. She's just wacky and unpredictable, enjoys committing acts of violence, has a tendency for getting out of tight situations you wouldn't expect her to, etc., but is who she is so you know precisely what you are getting in the next film.
To Marvel's credit, they certainly do let characters grow and change--e.g., Iron Man, Captain America, Hulk, Scarlet Witch, etc.
