The crew chief isnt necessarily responsible. There are very, very, very few drivers (one to be exact) who can clearly and consistently give feedback to the crew.
Earnhardt Jr. is not that good of a driver and as the competition level continues to rise he will continue to spiral downward toward mediocrity. His finishes were vastly above expectation for a good portion of his career, and he has since been performing at expectation, which is mid-pack.