I personally think it can work just fine with the mask on. But the problem is with the pom-poms. I think the way to make it look better is to make them massive and almost make them fall down like hair.
Girls with full masks on almost never look appealing in real life, because baldness isn't something we associate girls with. So you have to find ways to add the same aesthetic that long hair gives them.
Spider-gwen did this pretty well by giving her a hood. So I think by making Harley's pom-poms really long would fix this problem.
Pic related looks fine to me, because they fall down instead of jutting out. They could do with being a bit thicker though, but I think that's overall the way to fix it. Not with removing the neck area. But make them simulate hair flow instead.