That is the definition of populist, being popular. And in 1980 Reagan didn’t run on a strict conservative platform.
Reagan fell short and we still suffer for it.
Not even close. A populist is a politician that latches onto whatever issue is the most prevalent in the public mind, then crafts his position to take the greatest advantage of it. He has no real principles of his own, and as such is comfortable changing positions on a dime if the public sentiment changes. He is equally as comfortable relying on conservative positions as he is liberal positions, because the positions are not based on core beliefs, but are a means to an end - gaining power. That is a good description of Trump.
Reagan was popular - people like who he was and had respect for his as a person - but his positions were based on conservative principles. He didn't change his positions just go along with the public mood, but sought to bring the public mood around to the principles he espoused.