The Joker loves cracking on Batman's seriousness, but Britain's Batman is the exact opposite. While it might seem like the Joker would love a goofier Batman, the British hero Knight ends up sending the Clown Prince of Crime into an absolute frenzy.
While America's super villains are deadly serious, forcing its heroes to tackle their threats head on, England has a much different situation. In the DC Universe, England's villains are often far more goofy and quirky, leaving them not so much a threat as a periodic nuisance. This leads the nation's heroes to match that energy, often being just a goofy as their foes. Britain unfortunately has some of DC's most useless superheroes. DC's British heroes have such names like Salt of the Earth and Milkman while the villains take monikers like the Pirate Astronomers and Captain Moondfust. All of this even extends to the nation's own version of Batman and Robin, Knight and Squire. This dynamic duo has a Joker-like enemy named Jarvis. But he's not particularly dangerous, at least not until the real Joker shows up.
In Knight and Squire #6 by Paul Cornell and Jimmy Broxton, the Clown Prince of Crime arrives in England determined to show them what a real super villain can do. He disperses Joker masks across England that manipulates people into committing crimes, including murder. Specifically, Joker goes after England's goofy heroes by killing them in strange and unique ways. For instance, one hero dies by being pinned to a tin roof and then heating up the roof so much that it burns the hero alive. While Joker is foiled by his secret magic weakness, the acts he committed were still heinous beyond belief.
This is a pretty interesting and peculiar approach DC Comics has taken to its British superheroes. It's especially strange when compared to their Marvel counterparts. Marvel has Captain Britain, a serious superhero that is often a member of the British superhero team Excalibur along with frequent former members of the X-Men. Captain Britain's villains have exterminated the Avengers, so he's no joke. These British heroes face real threats and deal with them seriously, while Knight and Squire have adventures that are a bit more comical and relaxed in comparison.
Meanwhile, DC treats its American heroes, specifically Batman, with an extreme amount of grit. Batman is an incredibly dark and brooding figure. This is what makes him such a challenge for the Joker, who tries to get him to crack a smile in the name of humor. In fact, this is probably what gave birth to the Joker's catchphrase in The Dark Knight, "why so serious?" But when he arrives in England, he finds Britain's Batman to make a mockery of the original's legacy. His goofy gimmick is so insulting to the Joker that it makes him angry for the opposite reasons. Perhaps this is why the Joker's true nemesis is England's Batman and not Gotham's.