For me, the tone shifted around a hell of a lot. Like the start, you have the Floronic man basically mulch someone, bloodsplatter and everything, and then it goes into a goofy into song with cutesy art.
You have a weird fart joke that was actually kind of amusing because of Batman's "smells like discipline" response directly followed by a full song sung by two of twoface's goons set almost entirely to goons dancing. followed by Harley singing "hanging on the telephone" while gyrating around in an obvious fan service moment that kind of fell flat to me. But again, the entire song just set mostly to goons dancing.
Then there's the ending. I kind of digged Swamp Thing turning up and having this grand entrance, just to be like "Yeah fuck it, I'm not going to stop you" but then the weird ending of Batman and Nightwing kissing Harley on the cheek as they literally set fire to someone? It felt really out of character, particularly for Batman.
The humour was alright in places, but the story was dogshit, and the characterisation was crap and the tone was all over the shop. I guess that's because Harley is a madcap character and they wanted to have that run through the film like Brighton through a stick of rock, but it just didn't work for me.
At multiple points in the film me and the friends I watched it with said "what the fuck is the tone of this film" and "who the fuck is this scene meant to be for?"