Ignoring that City break FFP rules and have spent more since Guardiola arrived than any other club, Pep's had a couple of massive wins on the cheap in the transfer market. Jesus for £27m and, even with his injury record, £20m Gundogan is a steal.
But then he's also spent £80m on Danilo, Bravo, Nolito, Douglas Luiz, Moreno, Sandler, Zinchenko and Arzani who have either flopped/been sent out on loan/don't play.
He needed a new defence so, in addition to any of those just mentioned, has bought Laporte (£57m), Mendy (£52m), Stones (£47.5m), Walker (£45m) and Ederson (£35m). Works out at a total of £280m just on goalkeepers/defenders alone.
Mourinho needed a new defence but wasn't allowed to spend big on experienced players, so got teenager Dalot for £19m, as well as Bailly (£30m) and Lindelof (£31m) who were both 22. A total of £80m. Now the club are extending Valencia and Young's deals, two washed up wingers.
Unlike Guardiola, Mourinho hasn't had big transfer wins, so it's fair that his record is questioned. Although he's only signed 11 players in 3 summers. Zlatan was free and was great. Sanchez has flopped and replaced the biggest bottler Mkhitaryan, who had also flopped.
Lukaku had a good first season but is now the size of a house and struggling after the World Cup. 27 goals in his debut season for a team that doesn't go on the all out attack is a decent return. Pogba is the biggest buy and can be United's best then worst player on the pitch.
Matic has his moments. For £40m, we've not had a great deal (a better deal than Bakayoko was for the same price, but that doesn't help us). Spent a fortune on Fred who is "adjusting" but, in fairness, still far too early to judge. And 3rd choice keeper Grant. That's it.
Essentially, Mourinho hasn't done enough with what he's been given to prove he deserves more money. That said, he was only really backed for the first two years and even then, you couldnt argue he'd prefer Lindelof over a proven defender. Just did the best with what he was given.
And if the club doesn't trust him to buy the players he wants, then they shouldn't have extended his deal this year. What's the point in him being there if he's only allowed to sign one teenage full-back when he needs at least two more experienced players in that position?
If Mourinho had inherited a team with Aguero, Silva, De Bruyne, Sterling etc. then maybe he would've been allowed to spend all his money on a defence too. But United didn't have a midfield OR a star striker, on top of defensive problems. His only world-class player was in goal.
Mourinho has spent more than £40m on a player three times. Guardiola has brought in six players who cost £40m+, with Sterling (£44m) and De Bruyne (£55m) bought the season before. These were the players LvG bought the summer before Mourinho arrived. 
