Objectively speaking, The Batman Who Laughs could not be considered a complete piece of trash. When he first appeared, both his character design and abilities were fairly eye-catching.
It was a dramatic approach, because the Metal event at the very beginning needed something to grab readers' attention. A Batman who had killed the Joker made for an excellent gimmick.
To be fair, from the standpoint of comic art, The Batman Who Laughs had an exceptionally good visual design. They had not simply drawn a big mouth onto Batman in a Batsuit; they had redesigned his entire outfit.
Compared to Batman's costume, the Laughing Man's outfit was more revealing. It symbolized the shift from the traditional, classic knightly image to a modernist comic-book character, something that should have appealed to many readers who embraced neoliberalism.
After all, the rigid and conservative Batman had finally crossed his line and killed that detestable criminal. An eye for an eye, a tooth for a tooth—he had finally become what some readers wanted: not a hero who endured suffering, but an Avengers who took revenge on suffering.
Yet in the end, even those people rejected the new character. The biggest reason was that he was supposed to be a fusion of Batman and Joker, yet he contained none of Batman's or Joker's qualities.
The writers had created such an image and given him a premise, one that might originally have been grand and philosophically rich. But they overplayed their hand, turning it into a complete and utter scam.
Some people had looked forward to The Batman Who Laughs because they wanted to see the collision between the Batman who had killed Joker and the Joker who had been killed by Batman. It should have been an artistic and philosophical discussion, and DC readers loved that