everyone is a critic
For a hint of what went wrong, take a quick look at the CinemaScore numbers. CinemaScore polls audiences right after they've seen a movie to get a quick sense of the mood in the theater after a movie ends. "Bruno" scored a C.
While C is average at the schoolhouse, it stands for death at the multiplex. To create a frame of reference, consider that the vast majority of major summer releases score in the A to B range with audiences no matter how bad or good the film is: "Transformers: Revenge of the Fallen" received a B+ and "Ice Age: Dawn of the Dinosaurs" received a B; "Star Trek" scored an A, while "Up" scored an A+.