Mourinho should be ultimately held responsible because he was the manager when Pogba was signed. And it's always the manager's responsibility to turn his own signings into a success.
Having said that, I do think that Pogba's inability/unwillingness to do the basics expected of a player in the position he wants to play could well be the single biggest factor in our team flattering to deceive this season. He's our star player, someone who cost a huge amount of money, plays in the most influential position on the pitch and is supposed to be at the centre of everything we do as a team. He was even talking this up himself pre-season, when Thierry Henry was asking him what sort of player he wants to be and he basically said someone who can do absolutely everything. When he's consistently failing to track runners, giving the ball away cheaply, not making enough tackles, almost never breaking into a sprint and taking too much time on the ball then is it any wonder that we have only very rarely clicked as a team?
Sure, you'd like to think that Mourinho can "fix" him but it's also possible that he's an unfixable problem. As we all know, his stats at Juve were even worse than what we've seen at United. He was part of an extremely strong team but I suspect his involvement was mainly flitting round the edges of things, racking up tricks and flicks for highlight reels but generally needing all the hard work to be done by other players. The definition of a fair-weather footballer. His inflated fee was down to his excellent agent, his extreme marketability and the potential he had to grow and mature into a proper, well-rounded central midfielder. A potential that was, unfortunately, by no means guaranteed.
It's funny the grief that
@sammsky1 got for his "elephant in the room" thread because I think that's exactly what Pogba has become. The one player that - more than any other - has caused this team to malfunction since we signed him. And before anyone says anything, I know that we've also looked shit without him in the team. Arguably even shitter. But that's besides the point. The reason he's missed is because he does have
some good qualities, qualities sorely lacking in all our other CMs (creativity, specifically) but that's not worth the downside that comes with all the other flaws in his game. So we're damned without him and damned with him. Had the money been spent on a more well-rounded and disciplined creative central midfielder then our season would probably have turned out a hell of a lot better. Mourinho's to blame for not spending the money wisely but the uphill battle he's fought to get the best out of Pogba since then might be genuinely unwinnable.