They're not just smart, they're fortunate.
There's a funny trope that's still prevalent in anime, especially in shows that depict characters who accidentally end up in the wrong place (so, isekai, but not really): it's the "fake it til you make it" one.
The characters, as they try to fit the role they suddenly landed into, try to bluff their way out of some shenanigans, but their bluff ends up being right, and their lie somehow works out all the time. So we got a list of eight characters who demonstrated such behavior. They really got lucky, these ones.
Usopp (One Piece)
Usopp's imagination has no limits, his mouth running wild and generating stories faster than he breathes. But by some ill twist of fate even the most bizarre stories of his sometimes turn out real — for better or for worse. This is a truly magical ability, one that's not really officially documented but surely should be. Because it's hilarious and insane how his mouth sometimes lets the Straw Hat crew get out of the worst conundrums — or, sadly, lands them in it.
Ainz Ooal Gown (Overlord)
He's the president of the "fake it til you make it" club. A wizard who got isekai'd in another world as a ruler has to make some tough decisions throughout his reign, and his bluffing is one of the main comedic aspects of the show and the light novels. The novels dive deeper into that, adding more of his inner monologue and his inner panic that he feels upon talking to other characters, and also exploring the thoughts of those he talks to and how they interpret it — or, rather, misinterpret, but in a right way. Thankfully the arc that is being released now in the anime is one that's filled with this type of interactions in the light novel, so even anime-onlys will be able to fully immerse themselves in it.
Senku (Dr. Stone)
He's probably one of the only characters on this list who actually has enough brain power to back his actions, decisions, and revelations, but he doesn't only have intelligence on his side — luck also favors him. He benefits from a lot of coincidences, despite heavily relying on his huge scientific knowledge. This wonderful tandem adds a lot of hilarity to the show — but also keeps it a bit educational.
Kusuo Saiki (The Disastrous Life of Saiki K)
In Saiki's case, his abilities help him make his words true: he can warp reality. He's a psychic, and he tries to hide it, but sometimes he has to rely on his powers, and in these moments a lot of funny stuff happens, including some things that literally change the world in order to accommodate the lies that Saiki says in order to cover himself.
Sakamoto (Haven't You Heard? I'm Sakamoto)
Another reality warper — Sakamoto: a popular boy, a model student, swift and smart and perfect. Dangers don't bother him — he doesn't even notice them and they have no effect on him; surreal stuff surrounds him, but he doesn't care at all. The world adapts to him, not vice versa — that's his ultimate power.
Cid Kagenou (The Eminence In Shadow)
The protagonist of this series always wanted to rule the world, and after getting reborn in another universe he decides to be the shadow lord, manipulating everyone without being known. In order to do that he fabricates a huge lie that ends up being the truth, yet he believes it's a delusion. So he, being delusional but actually right, traverses through this world, battling the enemies and continuing to live this lie without realizing its reality.
Seiya Ryuuguuin (Cautious Hero: The Hero Is Overpowered but Overly Cautious)
The title says it all: Seiya is a hero with OP stats, but upon being summoned he ends up being very cautious of everything, up to the point of anxiety. So he trains, and trains, and learns new things until he deems himself worthy of an adventure that he has been summoned for. And, surprisingly, his caution pays off: he was right all along about the dangers that await him. His cautiousness, however, brought a lot of discomfort to Ristarte, the goddess that summoned him for the quest. But, thankfully, it was for the best.
Wein Salema Arbalest (The Genius Prince's Guide to Raising a Nation Out of Debt (Hey, How About Treason?))
Wein has to lead the country that he inherited from his father to prosperity. And he ends up doing so quite effectively. But most of the time it's despite his actions, not because of them. He loathes being the ruler, he wants to sell his country out and live a life without this type of responsibility! But no, everything he does coincidentally leads to restoring the country's wealth. His aid, Ninym Ralei, knows that he's quite smart in so many areas that can help restore the country and forces him into actually doing something good for it. And even his sabotage turns into something profitable.