It’s harder to get an apology out of people by calling them dummies. He’s the master negotiator, though; he must calculate that the likelihood of an apology is near-zero anyway. Might as well score some rhetorical points.
He's intentionally avoiding terms like con-men, sellouts, betrayers, and the criminally corrupt. So I guess using the term "Dummy" to describe folks in MSM/DC and Wall Street is PC, even for Trump.
Of course if he ever gets into the drivers seat, he'll have access to unprecedented data, classified intelligence, reports, files, backgrounds you name it. Only then will he be able to use those more accurate terms.