Leaving aside the appalling tactics, the generally miserable demeanour and negative public statements, and the fact that the team began last season really positively, and then Mourinho began playing negatively and the team never remotely touched those heights again...his record in the transfer market was dismal.
In 2016 - 17 Mourinho signed:
Bailly - hasn't been a complete disaster, but certainly hasn't been a success, and is now completely out of favour.
Ibrahimovic - hardly likely to be a long-term success, as he was nearly 35 when signed!
Mkhitaryan - left as a makeweight in Sanchez deal within 18 months, and completely frozen out of the first-team for some months.
Pogba - a qualified success, but has come nowhere near fulfilling his potential, or justifying his transfer fee.
In 2017 - 18 Mourinho signed:
Lukaku - doesn't look anything remotely close to a £90 million striker, and has got worse and worse the longer he has played for United.
Matic - another conservative signing, a stodgy midfield player who will be 31 in August.
Ibrahimovic - hey, why not re-sign him at 36!
Lindelof - a pretty decent signing, although there are much better centre-backs in the Premier League.
Sanchez - considered by some to be the worst ever PL signing, has been a total disaster.
In 2018 - 19 Mourinho signed:
Fred - has contributed almost nothing.
Dalot - shows some promise, but has a long way to go.
So he signed ten players, and I don't think any of them have been outstanding and emphatically successful. Pogba has had his moments, as you would expect from a world record transfer fee, Lindelof looks decent and could be good alongside a stronger centre-half. None of his other signings have been anything other than failures, and most of them aren't young and have zero potential to improve, and diminishing sell-on value.
Considering the other aspects of his managerial approach at United, he cannot wash his hands of all responsibility for what occurred during his tenure. There is no doubt that the club is not firing on all cylinders, but he was absolutely culpable for his own bad management.