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.