No, but seriously...
The phrase Mostly Harmless actually makes more sense than you think. Let me explain:
Right, so we all know that after coming up with the Answer - which of course was 42 - Deep Thought designed the computer that had the capacity to answer the Question - this of course was the Earth, which worked using its inhabitants as a kind of organic computer matrix. This supercomputer was specifically designed and set up to follow the evolution of the human race, starting with the cavemen.
However, when the Golgafrinchams crash-landed on prehistoric Earth, they inadvertantly buggered up the matrix by causing the cavemen to die out. And as Ford Prefect says, because the people of present-day Earth are in fact the descendents of these aliens, then the whole matter of the question is cocked up. Even when Arthur Dent tries to extract the Question from his brainwave patterns, all he gets is, “What do you get if you multiply six by nine?“
What all this suggests is that the supercomputer, and all its programming, were highly sensitive to interference from external forces - say, inhabitants of other planets. Obviously the matter of finding a Question to the Answer is of supreme importance, and the pan-dimensional beings in charge of it would go to great lengths to make sure nothing screws up the programme.
So what do they do? They deliberately engineer an intergalactic smear campaign against this insignificant little blue-green planet. They make sure Earth constantly gets the short end of the stick; they deliberately keep its technology primitive; they make sure humanity invents cricket - a mindbogglingly insensitive racial memory of the Krikkit Wars; they make sure the humans name a country Belgium (pardon my language); generally, they make it so that nobody in their right mind would ever want to visit such an unappealing and culturally insensitive planet.
Of course, after a certain point, the pan-dimensional beings hardly need to lift a finger - after the first few million years, anti-Earth sentiment becomes rife across the Galaxy, and after several generations pass, people start being born with an innate hatred for the planet. The only people who ever visit the Earth are either teasers stopping by for a laugh, or travellers who get stuck there.
Obviously the Hitchhiker’s Guide to the Galaxy absolutely has to have an entry on Earth, so how does one writer sum the place up in such a way as to make sure nobody wants to visit it?
Harmless.
And when Ford Prefect drops by to do a bit on Earth for the revised edition of the Guide, and ends up writing a rather comphrehensive entry, what do his publishers cut it down to in order to keep up the charade?
Mostly Harmless!
Of course, they needn’t have bothered, really. The Earth was blown up by the Vogons five minutes too early, and even if it hadn’t been, the Question would have been wrong thanks to the Golgafrinchams.
TL;DR: Earth’s poor reputation within the H2G2 universe was deliberately engineered by its designer and operators in order to protect the sensitive computer matrix contained within it while it worked out the Question to the Answer.
