Disagree man. He's a very good defensive fullback, bar the occasional brainfart in playing a striker on-side. His defensive stats are higher than most fullbacks of top teams because as a team we set up quite deep and defend a lot. I think he suits a Mourinho type team, hence Mourinho rates him.
However all other top 6 teams play with fullbacks/wingbacks quite high up the pitch. Their contribution is more offensive than defensive. That's backed up by stats as well. Chances created, assists and key passes per 90 mins... are higher for Walker, Trippier, Bellerin and Moses. Valencia is not very good at the offensive part. He's quite slow to get the ball out his feet and his cross delivery has regressed through the years.
He's better than most at the defensive part though. Problem with that is the offensive part of our game is where we're struggling. So the fan expectation is that we improve on that, particularly through the fullbacks playing higher and offering width to our attack. And Valencia is not the man for that, so he gets (part of) the blame.