Mostly Humourless, i'm afraid so. The fifth part of the Hitchhiker's Guide Trilogy is certainly the weakest and a sad way to finish.
There isn't any overriding plot till about half way through, even then it's almost inserted in just to finish the series off.
With no Zaphod or Marvin (there is an android called Colin though...) it's like an episode of Seinfeld without George and Kramer, it just wouldn't happen.
The ending is bleak yes but it's a proper ending and that's a small redeeming feature of Mostly Harmless but as a whole the series ends with a Big Whimper instead of a Big Bang.