He is a good player that has been disrespected simply because he was introduced by Mourinho. People won't admit it.
It's more complicated than that, though that is part of it.
Initially, many of the people who follow the U18s and U23s assumed he would be rubbish because other youth players (Ben Pearson, Rothwell, Goss etc) had looked far more promising, yet never got a chance in the first team and are currently playing, or sat on the bench, in the Championship. Most of these posters are extremely knowledgeable about youth players but predicting how players will develop between say 19 and 24 is very difficult. A few realised during the Sevilla, Chelsea, Liverpool spell a year ago that Scott was actually a decent player but some remained blinded by his underwhelming performances in the U23s. Even this season there have been plenty of posts along the lines of - Ben Pearson looked great for the U21s/U23s and he is playing for Preston, therefore McTominay must be Championship standard or not even that.
Then you've got the group of United fans that you mentioned, who assumed that once Mourinho left, McTominay would be sent out on loan or sold. Scott was only in the squad because he was tall as Mourinho favours tall players. McTominay was only getting starts due to Mourinho punishing Pogba. Scott was Mourinho's project so he could claim to have followed United's youth tradition.
With most of the players who come through the youth system, you seem to get a group of haters, who think United should spend £300m each summer on players in their mid to late 20s. United shouldn't develop our own young players. Even Rashford attracts this criticism, Lingard more so, Tom Cleverley and Fletcher before that. McTominay shouldn't be getting starts, instead United should buy Ndombele, Neves, Rice, etc. A super performance against PSG isn't enough to convince this group that Scott is a more sensible option than buying x, y or z. This week I read a post that suggested McTominay should be sent out on loan next season.
Finally, there's a surprisingly large group of posters who think Fred deserves a run in the team because he cost £50m, therefore he must be superior to McTominay. There's also a few posters who have liked Pereira for ages and are disappointed by his lack of playing time. Fred, Pereira and McTominay are effectively in competition with each other for playing time.
I forgot the posters who love technical ability and don't care about tedious defensive stuff. Last year there was a poster who was forever saying Lewis Cook of Bournemouth was a much better player than McTominay. Scott is a fairly unadventurous, hard-working, team player, of the type that probably doesn't get much credit at any club. He's going to have to come up with the occasional goal/assist to keep the abuse to a minimum.
It's not just this forum. It is really difficult to find balanced comments that give a fair reflection of Scott's strengths and weaknesses, the standard of his performances relative to the other players on the pitch. The main Manchester Evening News United writer Samuel Luckhurst absolutely hates McTominay, keeps going on about him winning the lottery simply by making his debut. He gave Scott an average performance rating for the PSG game (clearly in desperate need of a trip to the opticians). Andy Mitten and the other fanzine editors are somewhere between indifferent and brutally critical towards Scott. Stephen Howson the YouTube United expert (Full Time Devils), doesn't rate McTominay at all, despite being wildly enthusiastic about the likes of TFM, Axel Tuanzebe, Ro-Shaun Williams (now of Shrewsbury) and until fairly recently, Callum Gribbin. From time to time, a United expert on Twitter will admit that Scott is a better player than they had originally thought (only a year after it was blindingly obvious he was worthy of a place in the United squad).
Having written all that, the career paths of footballers can take surprising turns, so it is still possible that the negative posters will end up being more accurate than those who are currently optimistic about Scott's development. My judgement is far from perfect when it comes to predicting how players will develop. As for my bias, I'm biased towards players that I think will improve due to their age and personality, while being biased against players who are on the decline and those whose egos seem problematic for a team sport. So I guess that makes me biased towards McTominay, because he is relatively young, so expecting him to improve seems reasonable right now, and because he comes across as having a decent personality for a team sport. There's a few United players that I almost certainly criticise for any errors or poor performances, while rarely giving them much credit for good performances; next time Ashley Young puts in a good performance I must remember to rush to his performance thread to write something positive.
This post is way too critical of other posters. In most instances they simply prefer other players; nothing wrong with getting fed up with Scott's safe passing because they'd rather see Andreas Pereira play more often, or hoping we buy Rodri from Atletico Madrid, instead of giving Scott a run of games to see how he performs, before spending big money. Respect to anyone who initially thought McTominay was a bit rubbish yet posted to say they had changed their mind after seeing him play well in a big game this season or last year. The few times I watched Scott play in the U23s, I thought he looked completely out of his depth, playing as a make-shift centre forward as they didn't have any strikers that season. There are posters who seem to have misjudged McTominay yet called McNair and Blackett right, so their overall judgement appears perfectly decent. When a new manager arrives it is hardly a great surprise if several of the squad soon find themselves out of favour; it happened to Borthwick-Jackson, McNair and TFM when Mourinho joined.
Apologies to anyone who read to the end of this.