The idea isn't about being a generalist, but rather being valuable because you're like getting two okay guys or gals in one package.
Adams' own example is how he's by no means a highly talented artist nor is he a top comedian, but the combination of being halfway decent with a pen and having a better than average sense of humor suddenly puts a person into a much smaller group on the Venn diagram. And adding in just one more thing - his experience in the corporate business world, allowing him to create strips a lot of people could relate to - was enough to catapult Dilbert into a global phenomenon.
There are tons of moderately funny people in the world. And many okay line artists. And it's not hard to find someone with experience working in a corporate office. But the number of people who meet all three criteria is incredibly tiny. Heck, just having two of the three is quite rare.
The point being, it's far easier to become a big success by being above average in a few things, than it would be to try to be one of the best in a single area.
Finding a way to combine yout skills to make that success is the key, of course. And may require learning new skills or improving areas in which you're merely average.
One of the big themes of the book is how you shouldn't worry too much about trying new things and failing. For one, humans are terrible at anticipating what sort of work we would truly enjoy or be good at, and the only reliable way to find a true match is to try a lot of things and keep redirecting yourself. And for another thing, any skills you learn along the way only increase the odds of eventually finding a combination of skills that can lead to great success.
One thing many people reading the book overlook, I think, is that being mediocre in a lot of skills isn't the point. It can't hurt, of course, but the idea is to be above average in a combination of skills that can be utilized together in an interesting way. Recognizing that combination is more likely to be a process of trial and error rather than high minded planning.