The only person who could tell you what the author really wants to express would be the author himself. I have so far found very little source material where he expressed that (I'll try to edit in, if I find later) but apparently a book is soon coming out to publish many of his notes; perhaps there will be material then. (See: this article describing it.)
From the book itself, and material written by others about it, the book appears to be intended as a satirical commentary on the world we live in (at the time it was written, and much still relevant today). It points out, in extreme and satirical exaggeration, the negative/harmful aspects of bureaucracy (the Vogons, who are so attached to bureaucracy that it verges on cruelty, or perhaps more than just verges on cruelty). Technology, which sometimes does more harm than good (Douglas Adams has been quoted as saying, "Technology is a word that describes something that doesn’t work yet." (see here, and the first review I'll discuss below)), etc.
I found several reviews and such which discuss it, which may be of interest to you.
For example, from this article:
Evergreen insights about subjects such as bureaucracy abound. The
Vogons, for instance, are an alien race collectively disinclined to
save their grandmothers from certain death without orders “signed in
triplicate, sent in, sent back, queried, lost, found, subjected to
public inquiry, lost again, and finally buried in soft peat for three
months and recycled as firelighters”. As for politics, Adams noted:
“Anyone capable of getting themselves made President should on no
account be allowed to do the job.”
But Adams was just as likely to poke fun at technology. Critiquing the
need for multiple secure passwords, he creates a fictional
“Ident-I-Eeze” card designed to hold all of them; it is promptly
stolen. Plenty of other inventions go wrong in all-too-recognizable
ways.
Humanity’s overweening confidence in its own intelligence also gets a
skewering. Adams casts dolphins, for example, as a spacefaring species
more intelligent than humans.
Another example, in this review (emphasis mine):
One thing which pretty much everyone knows about The Hitchhiker’s
Guide to the Galaxy is that the ‘meaning of life’ (actually the Answer
to the Great Question of Life, the Universe and Everything) is ‘42’,
but there is also a much subtler answer to perhaps this very question
tucked inside.
When Slartibartfast, a planetary architect, tells Arthur that he is
particularly fond of the Norwegian fjords he designed, citing the
“little crinkly edges”, there is an almost Buddhist suggestion of
taking mindful enjoyment of the present, and a Bertrand Russellesque
view of fulfilling work and self-improving leisure. The dark side of
scientific materialism is also illuminated by an almost throwaway line
about the uncanny feeling that “relationships between people were
susceptible to the same laws that governed the relationships between
atoms and molecules.”
All that said, Douglas Adams was a humorist, and some of the book is meant to be just that. Trying to find meaning in the jokes, well, a lot of people have tried, and had a lot of fun with it, but you can't say the author intended it. Many theories were suggested by fans about the choice of the number 42. Wikipedia quotes alt.fan.douglas-adams, where Adams himself answered "Why the Number 42":
The answer to this is very simple. It was a joke. It had to be a number, an ordinary, smallish number, and I
chose that one. Binary representations, base thirteen, Tibetan monks are all complete nonsense. I sat at my desk,
stared into the garden and thought '42 will do' I typed it out. End of story.