Answer: Expectations are hard to benchmark
I'm honestly not sure there is a clear way to go about establishing what Trump "should" be winning, Mary. On the one hand, polls had him at 61 percent, so 60 percent of the vote (as of now) is pretty good — especially if you consider he'll win the nomination easily by mid-March if this is the case.
Another benchmark would be the percent of the GOP electorate that calls themselves "part of the MAGA movement." According to preliminary ABC exit polls, that's about 44 percent of SC voters (50 percent said no and 6 percent were unsure). So by this benchmark, Trump is actually overperforming.
Too bad there aren't a whole lot of modern examples of former presidents + failed presidential candidates running in subsequent primaries, or else we could try to establish historical baselines. Trump's performance right now is somewhere between George H.W. Bush vs. Pat Buchanan and Jimmy Carter vs. Ted Kennedy. Both of those incumbents lost in the general.
—G. Elliott Morris, 538