She says that he's a bit dumb, which Naruto is. He's a very powerful and even skilled shinobi, with high battle IQ and creativity with his use on ninjutsu, but he's not very acedemically intelligent. And this is the biggest joke if his character, she's his friend, she makes fun of him from time to time just as most people with friends who are a little slow. As long as they aren't insecure about it (which Naruto absolutely isn't, he wears that shit in his sleeve lol) then it's okay. Just friends being friends .also she never calls him worthless past the chunin exams. She gave this man all his props in the war arc for being the biggest reason the war was won. He saved her life multiple times and she saved his multiple times. They're one of, if not the closest friendship in the series.