There's lots of fingerpointing for this film. The big blame goes to the screenwriter, the director, and the film cutter.
The screenwriter - Douglas Adams - He should have known better. He did not produce a filmable script. He left in partial gags. (Half a gag isn't a gag at all.) He made changes to the story arc, but never fully committed to those changes. He did not boil down the story to a well-working whole. I generally like all the changes that he did make, because the entirely new or rewritten scenes generally worked far better in a visual medium.
Doug also had another problem: he introduced too much dramatic integrity, most specifically in the arc of Arthur-Trillian. In this case, dramatic integrity eroded comedic integrity. In many cases, this character develoment result in overexplaining jokes.
The director didn't understand the film. Period. Looking at his bio, he was just too inexperienced for this film. Fortunately, he did understand visuals and motion, which generally helped the sight gags and the pratfalls. For example, I very much liked the gag of Deep Thought watching a TV. The low-key delivery of that gag made it work very well.
The film editor had no idea of comedic pacing. None. Much shame goes to the lackluster editing of this work.
As for the actors, they could not deliver a joke between themselves. They were shameful. However, they did well in physical comedy, which showed in this film. The physical comedy generally worked far better than the verbal comedy.
All in all, this was a clumsy adaptation to screen.