I have to sorta disagree on that tbh.
Smart characters, when well written, have a certain expertise that is depicted in a realistic and coherent manner, have knowledge of the case equal to or lower than the actual audience, their deductions are based on sensible observations and a logical foundation for their conclusions.
Smart characters, when written poorly, will often have secret knowledge (I think Agatha Christie liked to do that?) where a pivotal element of the "Who dun it?" is unknown to the reader and in the end, the main character reveals the unknown information that acts as the central clue to reveal who the murderer was.
In terrible detective stories, you get convoluted explanations or things get merely handwaved.
Of course, the protagonis knew about the price hike for indigo-dyed silks in the markets in Lima 4 years ago.
Of course, the protagonist is aware of some secret infight in a noble family a generation ago over someone knocking up a chambermaid.
Of course, the protagonist has heard that one weird rumor from half across the globe regarding that obscure tidbit of crucial information that somehow unravels everything.
It's like they play with marked cards. But that's not how all such stories work.
One of my favorite episodes of Columbo has him working on a case of a poisoned chef and he keeps on badgering some restaurant critic (that the audience knows is guilty) that had dinner shortly before the chef died. Columbo just keeps putting pressure on the guy and it all culminates in a really awesome final scene, where Columbo and the restaurant critic cook a meal together and the critic sets up poison to kill Columbo. After Columbo reveals that he knows the critic was the murderer, we get this scene:
I really like that writing. The murderer gave himself away early on and it's something that when you watch that episode for the first time blows your mind. You don't pay attention to the critic when he shows up at the scene of the crime, but him not fussing over maybe being poisoned too reveals him as the one who did it.
That is very good writing, since Columbo makes a smart observation and logically deduces, researches and concludes. I love it.
Another example of decent writing, this time a scene from "Monk":
I mean, it's just a throwaway scene and it's not mind blowing, but Monk's deduction of the lawyer sleeping with his secretary is based on logically structured evidence that he found. Monk has savant-like memory and he's got a very keen eye for detail, but I woudl argue that's not the writers giving him "supernatural" perception, he's got severe autistic OCD, so things being a bit untidy (like pillows being the wrong way, an earing on the floor or something sticking out of a bag) are just things that would stick out to him like a sore thumb.
In general, his character has very strong abilities to notice fine details and remember them perfectly and this gets balanced out with a shitton of phobias, him being socially awkward, OCDs and so overall I would call him a "balanced" character.
The show overall of course doesn't hold a candle to classics like Columbo, but I always liked the scene I just linked, since it's such a concise example of structuring things in a manner that makes sense for the audience.
Bad writing in mystery shows oftentimes just blows away your suspension of disbelief.