A Graceling is a person who is born with an unusual power, a kind of extreme skill that sets her apart from others. Katsa, our protagonist, is a Graceling, so it makes sense that the title would refer to her in some way, but we don't think that's the only reason Cashore and her publishers chose this title.
Because as much as the book is about Katsa, it's also about all those other Gracelings who are isolated because of their abilities—the ones that are exploited by kings because their skills are useful to the court in some way, as well as the ones who are ostracized in their communities just because they're different.
The difficulty of standing out or standing apart; the challenge of breaking new ground or going against society's unspoken rules; the isolation we all feel because of our differences: these are issues the book explores, and they all pack up very neatly into the one word title, Graceling.