Depends on your perspective - Walker took a mediocre side and made them terrible, Martinez took a strong side and made them bad. Both nadirs in the club's history. The worst Everton teams I've ever personally seen have been under HKIII and Smith, but the raw material they were working with was just really weak, plus the club was basically falling to pieces off the pitch at that time. So you've got to take a balanced view of these things.
Walker calling it a day after us is a mystery - getting a team like Norwich to third in the league is not something you can bluff your way into, there has to be at least some managerial quality there. The way he failed with us though was like a man getting rumbled, like imposter syndrome except for real. It wasn't a failure of his system, or he had a bust up with senior pros, he was just a man way out of his depth and he basically owned up to that by calling it a day. El Gaffer takes a different approach.