I really wish more people had seen this, even on SHH forums, there are many fans who haven't seen this movie for some reason. All these years I have been hearing that "DC movies are too dark, they should have some humor, they need to be better received by Critics"
Well, guess what ? Both Shazam and Birds of Prey had humor and were colorful and light hearted, both movies were received well by Critics and yet both movies under-performed, because fans (and general audience) didn't turn up to support these movies.
The there was some asinine online campaign to paint this movie as Anti-Men, with SJW agenda (it's not anti men), or as Anti- Sonic (which makes no sense as it's target demographic is different, nit's not primarily aimed at kids, it has 'R' rating)
A sub- $100 mil US Box Office is not good but, since Harley Quinn is not a A-list character (Harley started as a side character in Batman The Animated Series in 1992, as Joker's side-kick, recently she's shown as independent anti-hero type character ), Harley Quinn despite appearing in animated movies, animated series and many Arkham games was never as popular as Marvel's Deadpool.
Most general audience know her from David Ayer's Suicide Squad movie, which was divisive and it was released 4 years ago.
We have already discussed the reasons why the movie is not doing well in US, the general consensus is, marketing, R-rating and the ridiculous title. Add to it the silly campaign launched by some people calming its woke or something. They tried this for Captain Marvel too but they failed, since MCU is too powerful brand and since the CM movie was hyped up by Kevin Feige as a must see movie before Avengers Endgame.