Yes and no.
It's possible for an author to portray a characters as highly intelligent without themselves being anywhere near the same level. That isn't even particularly hard, given that they not only create that character, but also create every other character and the world itself in their story. They know everything there is to know about their own novel, so it's not hard to help the "intelligent" character cheat a little and blur things up so they look almost omniscient. Clues to a murder can be fabricated, if your Sherlock needs something to spot. Who's to say there isn't some particular color to the stain on a shirt that leads down a deductive chain of reasoning to the killer's identity? Who can argue with Sherlock's observational and deductive skill when it's demonstrated so plainly?
There's an important distinction to be made, however, between a character that is portrayed to be highly intelligent, and an intelligent story. An intelligent, or rational, story is one in which the thoughts and behavior of characters, and the plot, make sense. It's a story in which characters behave rationally, or intelligently.
An author's ability to write rational fiction is definitely going to be limited by their intelligence.
https://tvtropes.org/pmwiki/pmwiki.php/Main/RationalFic
Note the initial quote regarding Sherlock Holmes: