Answer: Trump isn't doing that well, but that doesn't mean he'll lose this fall
For the purposes of this analysis, I think we should view Trump as an incumbent when measuring his performance in the primary. Republican voters know exactly what Trump is like in the White House, and they can judge for themselves how he did on the issues they care about (he generally does well on this axis) and his character (things are more muddled here).
If we think of him as an incumbent, he is doing quite poorly. Remember how Pat Buchanan embarrassed H.W. Bush by getting 38 percent of the vote in New Hampshire in 1992 and is blamed for weakening him for the general? Haley has been doing better than Buchanan across the early states. There are plenty of Republican voters who don't think of Trump as ideal. We've probably all heard their arguments. Something along the line of, "his character isn't ideal, but he fights for me."
Now, what does this all mean for the contest against Biden in the fall? Not much. The vast majority of those people will vote for Trump in the fall. And if we are sticking with the 1992 comparison, Bill Clinton was a popular candidate (net +11 favorability in September of '92). Joe Biden is not. His favorability rating is currently at net -15 points. So, Trump is doing poorly, but so is Biden. When it's a race to the bottom all around, doing poorly in a GOP primary doesn't tell us all that much.
—Galen Druke, 538