I agree to an extent. The club definitely didn't face stellar opposition in either of the cup runs and remained uncompetitive in the league. Van Gaal's record against the big teams was actually superior to Mourinho's too. I would say, though, that Mourinho's problems in the league often stemmed from a gobsmacking inability to actually put the ball in the net as opposed to Van Gaal's recurring inability to even manufacture chances.
Another difference worth mentioning is that Van Gaal's second season actually saw the club regress despite winning a cup - went out of the Champions League group stages against fairly mediocre competition, got beat by Liverpool in the Europa, scored far fewer goals than in his first term, managed to accrue fewer points and so fell from fourth to fifth in the league. There was also an apparently better manager ready and waiting to take over.
By contrast Mourinho is currently 2nd in the league, the team has already scored more goals than in Van Gaal's final season and is on course to concede fewer. He's advanced from the group stages of the Champions League and remains in the FA cup. His points total, goals scored and position in other competitions all point to a steady improvement both over Van Gaal and his own first term. Certainly the team is still performing below my expectations and if things don't continue to improve then I agree that he will no longer deserve stronger support - as things are though he retains my tentative confidence.
I can be behind that and tentativeness summarize my feelings, I want to believe in him but I'm not sure if I should.