The pitcher is mostly responsible for his ERA, but how much of it he is responsible for depends on several things.
Where does he pitch at (Petco and GABP are awfully different)?
How many bats does he miss (more strikeouts means less chances for other things to come into play such as the ballpark or the defense)?
How is his defense, and in particular his infield or outfield defense depending on whether he is a groundball or flyball guy?