I'd say a whole lot of authors don't have what it takes to write a character vastly above their own level of intelligence. Of course many do manage to do it by smartly avoiding all the details they know they couldn't pull off depicting, some perhaps invest time in doing deep research and maybe even consult some expert. However, too many authors simply take the ass pull route, which always falls short. So, yeah, a mangaka/LN author obviously doesn't need to be as intelligent as their characters, but they need to be crafty enough to avoid pitfalls. Unless it's an isekai author. Because nearly all isekai is trash and nobody cares.