More of an intentional choice they made, but Michael Cera's portrayal of Scott Pilgrim in the movie.

In the comics, Scott was a nerd, but was more of a jerk than awkward. There were a decent amount of flashbacks about Scott being the best brawler in Toronto and beating up entire school's worth of "bullies". Effectively a nerd that got tired of being bullied and started beating up people in response.

The comics don't skim over Scott's shitty behavior, and he gets called out on it far more frequently, keeping that aspect of his character fresh.

They also don't skim over Ramona being a mess either.

In the movie, Scott is awkward and nerdy (Cera's typecast at the time), and it's played like he doesn't know what he's doing is wrong (dating a highschooler as an adult, cheating on her with Ramona, beating up the person she leaves him for, etc).

Ramona's issues are skimmed over for the sake of her being a stereotype "manic pixie dreamgirl" rather than a deeply insecure woman running from each and every mistake she's made over the years.

So it's more of a case of "intentional streamlining and dumbing down character traits to fit into the expected romcom roles" than cool character being ruined by the actor, but at first watch it felt like the changes were all from Cera's performance.

Brie Larson's Captain Marvel, although after seeing clips of her performing in non-MCU stuff I will die on the hill that those performances were bad mostly due to poor directing

Jim Carrey in "The Number 23". It was a rare serious role for him where he was supposed to be this sort of noir edgy tough guy. The more serious he got the more comical the whole thing seemed lol

Poochie the Dog.

Vanilla ice in Cool as Ice...Am I suppose to like this guy? He seems like a complete jerk. Sly Stallone in Cobra is hilariously over the top trying to be cool. Joe Piscopo in Dead Heat. Diceman in The adventures of Ford fairlane. Love some Chuck Norris/ Steven Segal movies but they often make the characters less cool. Hallie Berry in a lot of movies: X-men as Storm (who is a f.ing goddess and awesome character!) Catwoman, John wick 3, die another day, etc. George Clooney in From Dusk till dawn. Cage in Drive Angry. Too many Video game / comic book movies to mention.

I can’t think of a specific example. But this is gonna be something from a lame action movie where the protagonist is a ‘total badass’ but the script is so shit it simply doesn’t land.

Most of the Taylor Sheridan shows and movies. I don't know if he writes the dialogs, but there is a lot of cringe lines and scenes.