Mbappé opened the scoring from the penalty spot in the 11th minute, Bellingham doubled the advantage ten minutes later and Diomande completed the scoring with a spectacular strike in the 36th minute.
Racing Santander coach José Alberto felt the opening penalty had changed the course of the contest, while accepting that a later VAR intervention to overturn a handball decision in his team's favour had been correct.
"We are facing a great Madrid, a Madrid that dominated us in the first half-hour and was much better than us, but I think that first goal conditioned everything," he said.
"What I don't understand is why there are matches where VAR intervenes and others where it doesn't. There are incidents where it does and others where it doesn't."
José Alberto was particularly unhappy that the officials did not review the challenge that led to Mbappé's penalty.
"Everyone who understands football watches that action and nothing happens. Football allows contact. What you cannot do is bring someone down or hold them, but you can touch a player. There is no holding, no push, absolutely nothing."
He also praised the decision to overturn a separate penalty awarded to Racing after a handball, describing it as the correct use of the technology.
"That action they review, the handball penalty they take away, for me it is correctly interpreted. That is what VAR is there for. But the first penalty, everyone who knows football can see that absolutely nothing happens."
Mourinho, meanwhile, was satisfied with Madrid's performance and the way his players approached the match.
"It was a 0-4 game, but it could have become complicated. It is a good victory in an important week," he said.
"I have seen a very committed team. The team is growing. We have to score more goals. Against a low block, the team knows how to stay compact. The message of sacrifice is very positive."
The Portuguese coach also highlighted the growing understanding between his ideas and the players' qualities.
"We are growing. We are not at 100 per cent. It is a progressive development. We understand things that we didn't understand on the first day. The human empathy is good between us, my ideas and their qualities. We are doing well."
Mourinho dismissed the suggestion that Aurélien Tchouaméni's involvement in the penalty controversy was a foul.
"It is never a penalty for Tchouaméni. From 60 metres I couldn't see it, but when I watched it I was sure the VAR wasn't asleep. It is not a penalty. The referee was technically very good."
The victory extended Madrid's winning run to six matches in all competitions, giving Mourinho another positive result ahead of a demanding week.