In the first movie Batman's suit stopped bullets
In this movie it's just foam latex, as seen when he tears his mask/hood off, and when Catwoman scratches through it. Of course, the prop costume has always been foam latex, but, in-universe, how/why did it go from doubling as body armor made from some unknown high-tech material in Batman (1989), to just being a rubber Halloween costume in Batman Returns?
It's no wonder that Christopher Walken's character thought Bruce Wayne was a Batman imposter; he just saw him reveal that his suit is nothing but rubber.