Really, from my understanding, you're right: it should not make sense. It's pretty much an absurdist book, it was adapted from a radio show and was more or less just comedy sketches centered in a ragtag, nonsense setting. None of them really have much to do with each other, nor do they really build off of each other, but I find Douglas Adams writing and ability to come up with what he does to be endlessly engaging and uplifting.

You mentioned judging the scenes by themselves; this is how I view Hitchhiker's Guide, as a compilation of great scenes that each occasionally do have some great meta-commentary on the world at large, and almost all are funny and self-contained. There isn't so much "development" as there is a progression of in-jokes.

The characters are more or less used as puzzle pieces to fit into a scene; while they might not really "develop" so to speak, they are fun archetypes that are thrown into silly situations, and we recognize them by what they do and say. They don't compare to most novels or literature when it comes to character development, but it can often be a gag as well just how far they've come without developing whatsoever.

It definitely does try hard, and it definitely is incoherent. This turns it off to a lot of people, and that's totally reasonable. For those who essentially want to read Spaceballs, however, it is a work of art comparable to Plato's Republic.