He was RB6 so I'm not sure I would say he was a league winner.
As to who is at fault like I said the line is doing him no favors but he is doing absolutely nothing to help. This whole situation reminds me EXACTLY of the same discussions I was having as I bailed on Leonard Fournette a year and a half ago. Much like Fournette then, every conversation with Mixon now starts with "well obviously he's one of the most talented backs in football, but...".
With Fournette I asked people to show me what he does on the field that makes him one of the most talented backs in football, and no one could ever answer it. I see a lot of the same with Mixon.
Why are we so sure he's one of the most talented backs in football? Because everyone thought he was talented 4 years ago when he was a draft prospect? I have Mixon in the same high stakes league I had Fournette in so like Fournette I watch probably every snap he's taken the last few years. He does NOTHING impressive. He doesn't make guys miss. He doesn't run particularly strongly. He is putrid at the goaline.
The biggest thing he's missing is he has absolutely zero vision or creativity at the 2nd level. Nick Chubb isn't running behind a good line but he makes the most of his opportunities. When he gets to the second level he runs through an arm tackle, makes a sick cut to put a DB out of the play, takes a good angle and turns a 7 yard run into a 40 yard TD. Mixon gets into the second level and kind of just runs straight into 3 guys and falls over. That's the biggest thing missing in his game relative to other RBs behind bad lines. He just completely lacks the ability to make a special play when given the opportunity, which is weird to say given that he was exactly that kind of player in college. It just hasn't translated to the NFL.
One thing you will find lacking in Mixon threads with the people defending him is descriptions of how he is making these incredible runs just to get back to the line of scrimmage or gain 3 yards. We've seen that a lot with good RBs behind bad lines. They make a sweet spin move, drag a linebacker, etc just to gain 2 yards. Mixon never really does that. There are never these plays where you're like "wow, that was amazing to even get back to the line there".
I think he is what he is. A slightly above average NFL RB. He would be fine on a good team. Maybe the Bengals can become one with Burrow now in town and Mixon can be a decent fantasy asset. But he is simply, absolutely not this uber talented elite RB that people have treated him as for his entire career. The only guys that I can ever remember holding his perception of being an elite player without actually doing anything elite on the field for this long are Leonard Fournette and Sam Bradford, and it didn't exaclty work out for those guys.