You know, when I first read this, I thought you were talking about the sequel. And I thought, that is one huge spoiler. Could be quite a twist for the second movie, if Harley was a hallucination!
I really liked the first movie, even though I don't think it really feels like the Joker character from Batman, it's more like Taxi Driver.
I hear the second movie isn't doing well at the box office. I blame Gaga.
In a wierd way it is a spoiler (ie apologies for not saying "spoiler alert") for the first Joker movie. However, I don't feel the hallucination thing is a true spoiler because it is a nuance you might only pick-up if you re-watch the first Joker movie a few times or are really into that. If you are there just for the action rather than going in very deep, it's actually a plot-point that people would likely miss or not care that much about. It's a more of a "whatever" plot point than anything serious. If anything, this might make it easier to understand on the first viewing.
I actually missed it on my first viewing of the movie. I had to look at a video movie review, after the fact of wathcing it the first time, for it to sink in that it was a hallucination, because it didn't even click. The movie really has allot of moving parts and it's a nuance that will be missed on a first viewing, even though I'm making it sound serious on paper.
In terms of the Joker character, it's one of the most romantic anti-hero portrayals of that character. It's like you feel sympathy for the Joker character and feel he's like a unfortunate incel guy who had a rough life, allot of set-backs, that snapped and turned mad and morphed into that character. That's why there were warnings about the movie when it was played in cinema.
Personally, I think the Gotham TV-show has the most interesting portrayal for the Batman world (ie Gotham city). You are seeing younger verions of everyone on that show. I felt the best connection to Batman from that show. Movies, like Joker (2019), also inspired interest into this with me because you feel a connection with that character as someone who is for the underdogs. The Christopher Nolan, while he has intersting movies...I was never really "into" his Batman/Joker portrayals. More like a fun action movie with me then anything deep.