Despite its success, Joker still faced significant criticism. Among the main issues highlighted was the brutality of its violence, the darkness of its tone, and its portrayal of mental illness. David Fincher claimed Joker betrayed the mentally ill, and his criticism wasn't entirely inaccurate, even though the matter is still debated. The fact remains that Joker's depiction of mental illness was largely divisive, and the film's somewhat ambiguous ending only served to compound the problem.
The first reveal of Joker 2's story details indicates that the setting of Arkham Asylum will play a large role in the sequel, but with that tease comes the risk that Folie à Deux will repeat Joker's mistake. The first film's approach to mental illness is considered divisive at best, but setting the sequel inside the walls of Gotham's infamous mental health facility could very well spell an even more problematic approach to the topic. Joker 2's Arkham story reveal is an interesting prospect, particularly as the film will reportedly be a musical. Still, when considered against the criticisms leveled at the first film, it may indicate that Folie à Deux's story will be equally problematic.
Mental illness is an incredibly sensitive subject, so delivering a grounded take on the Joker's origin story was always bound to be divisive. Ultimately, there remains a contingent that considers Joker's depiction of mental illness problematic, and Joker 2's story reportedly being set within the walls of Arkham Asylum may well compound that issue. Of course, the film's title refers to the Joker's "madness" - Folie à Deux literally means "madness of two" - and that terminology alone is considered problematic in certain circles.
What's more, Joker: Folie à Deux will also introduce a version of Harley Quinn to Phoenix's Joker, and the pair's relationship is usually shown to be built upon their shared delusions and mental illness. This makes it even more likely that Joker 2's story will prove every bit as divisive as Joker's because it will undoubtedly feature even more characters who live with some form of mental illness. There's also the unfortunate subtext that Arthur Fleck's mental illness is what causes his violent criminal streak. If Joker 2 doesn't address the idea at all, it could actually retroactively make the Joker criticism worse.
Interestingly, Joker 2's musical storyline could offset some of the more troubling and problematic elements of the film's story by further removing them from reality. Joker was a very grounded take on the titular villain, but introducing a musical element could help to establish a greater separation from the real world, which could act as a barrier to the more troubling interpretations of its story. However, it's essentially impossible to make a movie about a supervillain whose entire gimmick is "madness" without touching upon the controversial subject matter. Therefore, Joker: Folie à Deux will simply need to tread carefully in its approach to its Arkham Asylum story.