How bad an actor is Aiden Gillen?

There are far worse actors. Aiden, on the contrary, is quite brilliant. I kinda agree with the overacting sometimes further in the series, but he IS Petyr.

I think he's genius and makes the character absolutely lovable. When he opens his mouth, he can melt even a White Walker's heart.
I'm sorry you feel that way about Aidan, but he defo does it for me.
Concerning Iain Glen (Jorah) you might be right or maybe it just appears that way because both their characters had far less screen time lately.

The first thing you have to remember is that Littlefinger is the most complex character in the books and series and there is hardly any proper description of him save the POVs of the other characters. Aidan has to work with clues that are scattered about in the books and how he is portrayed in the script. So with what he has been given, I think he has done a wonderful job. The season 1 portrayal of Littlefinger was exactly as in the books but post that, the writers turned him into the usual villain. All the actors can only work with what they've been given, but I believe Aidan has done it well and he has improved on him considerably (yes I am referring to his accent). Not only have the writers turned Baelish into your everyday villain, they have made him into a foolish one too. But I honestly believe that the actor has portrayed the character well.