I don't think Adams is talking about how reluctant the hero is but about failure and misfortune.

Frodo definitely doesn't want to be there but he is far from being a failure. He saves Middle-earth, goes home to the Shire and saves that too, and is regarded as such an incredible mortal that he's invited to live in Valinor with the elves (this is a very big deal and I believe has only ever happened for Frodo and his buddies).

The same goes for Harry Potter. He's a loser at the beginning but after going to Hogwarts he's very much a hero that saves the day by being good at everything and exceptionally brave.

Also, I'd say there are plenty of reluctant heroes in American literature and film. Luke Skywalker hesitates to go save Leia in the first film, Spider-man straight up quits being Spider-man multiple times, John McClane just happened to be there when terrorists attack.