... was Harry Seldon correct? .. is Donald Trump the anticipated Mule?
I think you are misremembering the Foundation series.
Trump is not an outlier who upsets the fundamental assumptions of our political system. He is simply the distillation of racist/sexist/xenophobic/anti-intellectual threads which have been part of the American body politic pretty much since the country was founded.
He is the (political) reincarnation of Joseph McCarty, Barry Goldwater, George Wallace, and so forth, not a mutant with unusual mind control powers (as The Mule was in the Foundation series).
If we truly did have a science of psychohistory, the outcome of this election (which was VERY narrowly decided. HRC won the popular vote and a shift of less than one million votes sprinkled between several narrowly-won states could have reversed the result) would not be so unexpected.
In a time of great anti-Establisment opinion when the media has evolved to cover elections as "Reality TV" horse races, a man who spouts a strong line of anti-Establishment rhetoric and who understands the mechanics of Reality TV runs against a politician who is the epitome of the Establishment (and many of its excesses). The outcome isn't terribly surprising.