Like Hank himself, the title gets to the point. There's a Yankee. From Connecticut. And he has adventures in King Arthur's Court. Clearly, Twain didn't want anyone missing the point. On a slightly deeper level, the title suggests something absurd and kind of funny about it all… helping readers understand what was in store—an American in Camelot is a pretty silly premise, after all.
Finally, the title also points out the contrasts upon which the whole narrative is based: practical level-headed Hank versus epic over-the-top Camelot. Hank is at odds with his environment from the get-go, and only one will emerge triumphant. How and why, you ask? You'll have to open the book to find out.