The problem, it seems to me, is that the cover is trying to be dead-serious and scary, on a book that isn't known for that. You can do this concept and actually play it a little more... jokey, I guess, instead of Joker-y. Have her giving him the side-eye and a skeptical look, with her arms folded, and turn the lights up a little bit. Have a stupid fuckin' "bang!" flag sticking out of the gun he's draping over her shoulder or something like that.
I mean, the Superman variant is spoofing "Can You Read My Mind" from Superman: The Movie - and that's a book that doesn't have the tone Batgirl does for the most part. So why is the Batgirl variant an intentionally disturbing reminder of how Joker FUCKED her up back in the 80s?
This is an image that wants to be nightmarish and twisted, attached to the front of a book that isn't that at all, referencing a part of the character's history that not even the writer of that specific event (Alan Moore) thinks is very tasteful.
It's a more lighthearted sort of book - the image should reflect that. Joker doesn't HAVE to be the malevolent psychopath he CAN be in this image. He can be goofy, and he should have been. There's a way to pull off this idea in a much more goofy way, and that it wasn't even attempted is a problem, and why you have some of the people who read the book and care about the character reacting to the variant cover in the way they are.