I agree that it may not be as funny as some of the other Hitchhiker books, but then again, books 3 & 4 (Life, The Universe... & So Long, and Thanks...) were already less funny than the first two, which both really stand out above the rest on the humor scale. Partly because of its sheer random plotless road-movie style.
To me, books 3 & 4 were the ones that suffered from lack of plot/satisfying ending. Especially So Long and Thanks... was, though quite funny at times, rather a disappointment in the end, though it started off very well, a bit in the style of the Dirk Gently novels. He might have apologised for the inconvenience indeed.
As it is, it seems to me that, steering further away from the absurd humor that inhabited the beginning of the series, Adams tried to write out a good plot (a bit like with the Dirk Gently novels) that would satisfyingly wrap up the whole series - tricky, but could he do it? Yes, definitely yes. I can readily say that the "trilogy" wouldn't have been complete without it! It is a pity that he didn't hold onto the meandering nutter-style. Note that the book chapters switch very orderly between Trillian/Arthur/Ford, as do most of the more conventional novels. That's because here, he's more interested in creating a mystery with suspense and tension, rather than following in the footsteps of Monty Python. That is, the general plot here still makes absolutely no real sense (though everything fits in the end), but there are not much absurdities in the story itself, and the dialogues are less important and contain less unforgettable oneliners - DA concentrates on telling the story and finishing it.
Maybe Adams was better (and probably unique) at being an heir to Python rather than being a detective/mystery novelist. Still, in picking a totally absurd idea and working it out in such a way that it wraps it all up in a satisfactory manner is some stunt. Where the original two books get 1 star for plot and 4 for humor, this one gets 2 for humor and 3 for plot - still adding up to 5!