When my son was at primary school there was a girl in his class who was good at everything. She was in the first team for netball and played county level tennis. She played the violin in the National Youth Orchestra. She was in the top sets for every subject. She was a very clever little girl, and loathed by everyone.*
It's hard to like someone who is so clever it all appears easy, and the same is true for characters. Detective writers have to get round the cleverness rule because detectives by definition need to be brighter than the readers so they can get the answer when we are still floundering. There are several ways to do it. Sherlock Holmes, for example, is not the narrator. That's Dr Watson, who is amazed at his friend's cleverness. If Sherlock Holmes was the main viewpoint character he'd be sneering at all us dimwits who couldn't see what was so obvious to him. Plus, as he'd get the answers in double quick time, the stories would be very short indeed. Because they're told from Dr Watson's point of view they last until Sherlock reveals all to him.
Other clever detectives have major character flaws - alcohol, gambling and social ineptitude are popular. They may be clever, but we're glad we're not them. Hercule Poirot is Belgian. Others make mistakes and go down the wrong path before coming to the right conclusion - I've always liked Inspector Wexford because he seems so ordinary, not intellectually clever but full of common sense and homespun wisdom.
Outside detective fiction, if main characters are high flyers, they often get brought down to earth by a more human element - think of all those fictional career girls who get babies dumped on them. They may be good in the boardroom, but they're useless with a nappy! Show me a scientist and I'll show you a character who is domestically incompetent. Call it Tall Poppy Syndrome or what you will, but very clever characters do not make heroes.
If they have to be clever to make the plot work, then they need time to come up with the clever solution. When Milo solves the impossible problem in The Phantom Tolbooth, the author (Norton Juster) specifically tells us that he'd "thought about this problem very carefully ever since leaving Digitopolis." Dick Francis made a career of writing about ordinary, unremarkable blokes who get swept up into exciting situations and manage to make the best of it. It may hurt to suppress your own natural genius, but your main characters need to be of mainstream level intelligence, just like the readers.
*Poor kid, it wasn't really her fault she couldn't make friends, we tried inviting her to tea but were told by her mother that the child couldn't come because she had a full schedule of extra classes and practice every evening. I hope later on she rebelled.