IMO, Pilou Asbæk as Euron Greyjoy being awful had nothing to do with the actor and everything to do with the writing. He actually even resembles Theon a bit, but when the writing for the character is that bad there's not much anyone could've done with it.
A worse casting choice in the Greyjoy family was Asha (Yara) Greyjoy. She looks and acts almost nothing like her book character.
I'm going to answer this response and possibly open up a firestorm of controversy by stating that many of you are operating under a false assumption.
My impression is that actors, particularly ones that aren't like the most well known in the world must audtion for parts.
Then on the strength of those audition, someone calls them back to read again or because they have the part.
So someone makes a decision who is not the actor.
Casey Affleck just won best actor at the Golden Globes. He would not make a good Tywin Lannister. This is obviously not his
fault.
But just because an actor isn't at fault doesn't make something a good casting choice.
"No one could have made that garbage good." also doesn't work. People make garbage good all the time. that also doesn't make them the best choice for roles.
Steve Buscemi played a great small time criminal in the movie Fargo. I also loved his performance as a pathetic loner in Ghost World. I thought his performance was rather underwhelming in Boardwalk Empire, and he had tons of material to work with over 5 seasons.
This isn't about assigning blame to actors specifically, but about assigning blame to choices. Someone made a choice and it resulted in an actor playing a role. What followed was what it was, and it's done.