I can copy and paste what I had down it turned into a write-up of sorts but here it is...
Why can Harley do what she's able to do?
Watchmen's hyper-realist approach imposing the impractical notion of a Superhero
(parodying various tropes that come with it) into a practical realistic setting I see as an artistic portrayal of the unreal imposed upon the real. Ultimately to serve the narrative, various characters are able to do what they're able to do in a way that makes sense in a mind over matter kind of way even though of course in reality what they do with the power sets they have would be sure to fail eventually...
The way I see it is Gotham City is best represented from an artistic standpoint as a fantasy realm evoking a certain architectual/artistic vibe as summed up here as ranging from Art Deco to Dystopian:
The Gothamist Movement | Hazlitt
Within this fictional playground, I think Batman stories best work without trying to be too realistic like the Nolan films but moreso utilizing a logic or standard that simply mirrors a realistic setting. In terms of the mundane/everyday life in Gotham, realism seemingly holds up. Other elements such as a writer trying to assert what characters should be capable of what from a realistic perspective just wouldn't work for anyone including Batman whose merely human though. If this were the case, Batman would be dead, arrested, or forced to flee fairly quickly even if some success initially... A writer trying to assert otherwise just strains my patience too much as a viewer or a reader.
Harley is able to do what she is able to do because it's mind over matter. The logic/standard is the mundane everyday citizen suddenly going off track with things suddenly spiraling out of control where their reality becomes part of this seemingly realistically modeled fantasy realm of Gotham City is the story logic in a narrative that recurs over and over again. Of course, in real life abilities are contained by physical limitations but in Gotham this is a narrative often encountered as I found summed up here:
I found the following art that echoes the notions pertaining to the everyday realism of Gotham amplified into perhaps what might be considered amplified realism into a fantastical state:
Letter to Batman – The Making Of:
19th Century Painting "Reply of the Zaporozhian Cossacks" by Russian realist Ila Repin
Digital Fan Rendering:
Hyperreality:
The above quote applied to fiction reminds me moreso of the movie the Matrix but could be applied to the comic reality that is Gotham or the world pertaining to Watchmen.
A breakdown of the controlled structured life of an average citizen of Gotham spiraling into a delusion allows the impossible to suddenly become possible as just another narrative thread of the typical type of hero that must try and prevent the typical type of villain from taking over the city in this fantastical realm that is Gotham.