Harley’s redemption arc doesn’t make any sense because her “power” is just being criminally insane. She doesn’t have any real powers that would lend themselves to overcoming other bad guys. A “redeemed” Harley Quinn is just an upstanding citizen.

It’s like how having the power of audacity empowers you to be an asshole. Fixing you doesn’t turn you into a super hero. It just means you’re not an asshole anymore.

This is why good guy Harley Quinn functions mostly like a cartoon character. Otherwise there’s no meaningful advantage in wielding a baseball bat against evil Superman.