The only way a character does not need something that is streamlined to the rest of the cast is if they are so strong that it is an afterthought.. All your thoughts and logic place her solid mid tier.. Nothing about that is strong.
Well, I'm of the 'high tier chars should be nerfed to mid, low tier chars buffed to mid, and mid tier chars should stay the same' school of thought - I'm aware that others prefer 'everyone else should be buffed to be as good as high tier chars', which I guess is fair enough.
She has average/basic zoning tools, sure she has great strings and combos if the opponent doesnt know how to block low...At the end of the day.. A good Harley player can make an average player look baaaaaaad... But you are never going to see a Good Harley player make another good player look bad..
Edit: by the way.. Who is your main?
Yeah, her lack of solid overheads is the main reason she's not high tier - give her Wonder Woman's b2 and we're good to go.
Seriously though, Harley Quinn has some pretty decent shit going for her. 2b3 mix-ups can be excellent depending on who your opponent is using, b22 even aside from being a great combo starter is fast and has great frame advantage on block, f23 (which in fairness is easy to revert to standing block on reaction if they know the matchup) likewise has solid frame advantage, and her strings can be cancelled into db2 which, while punishable, you can cancel into a backdash and keep them guessing.
And her MB specials are great too - MB cupcake is obvious, and MB Play Doctor adds huge damage onto her combos. I just don't think she needs buffs especially (not any big ones, at any rate)
If you're interested, my primary character of choice is Raven, though Harley is easily my 2nd/3rd most used.