Some are saying that if he is a hero, then surely he is one only in the sense that he is the story's protagonist. But what I think they are all forgetting is that one of the universal archetypes is that of the fool.
It's one of the cards in the set of Tarot cards proper, surviving in a deck of playing cards as the joker, no less. I mention this to highlight the connection to comedy.
The Fool in the classic sense is no loser, though he does not necessarily exit all his battles victoriously. He is directionless, never choosing where he will go so much as having his direction chosen for him. This inevitably leads to adventure (and misadventure). Great things happen in his wake, and great people often seem little more than props in the tale... some even recognize that they are such. And when the "right thing" happens despite the fool's ineptitude, it is often the case that this is so because of the fool's heart, his capacity for selflessness, you might even say it's because of his wholesomeness.
Does this sound familiar?
I contend that Arthur Dent really is a hero in the proper and classical sense.