Yes, in some ways it can be a common statement. Naruto has a huge and passionate fan base, and fanfiction related to it is abundant. Many fans create their own stories within the Naruto universe, so in the context of the large amount of fan - created content, one could say Naruto is a source for a great deal of fanfiction.
Well, it's sort of common. The Naruto franchise has such a rich world with complex characters and an elaborate plot. Fans are drawn to it and like to put their own spin on things. Fanfiction gives them the chance to rewrite parts of the story, create new relationships between characters, or even give minor characters more screen time. Since there is so much of this fan - made content out there, the idea that 'naruto is a fanfiction' isn't that far - fetched as it shows how the original work has inspired so many people to create their own versions of the story.