That feels like a more or less willful misreading.
Let's suppose that my friend crosses paths with 100 Iowans during a business trip there. Let's suppose that 1% of Iowans are racist or bigoted enough to at the very least yell some abuse at this particular clearly non-caucasian individual. Before Trump's primary campaign and electoral win, social pressure kept the 1 person my friend may have encountered from acting on that impulse, but afterwards, that person felt much freer to just speak their mind (or worse).
Such an analysis says nothing about Iowans in general as people. It says that it's easy to be worried that the general attitude had shifted enough in Iowa (but not, say, in NYC or Chicago) that the tiny percentage of people who might act on their racist/bigoted impulses are now more likely to do so, changing my friend's risk exposure by visiting.
It's not for you or me to judge whether that's sufficient reason to avoid Iowa now. That's an individual decision to be made, and I do not sit in judgement of those who decide one way or another.
But you're also missing the deeper aspect of the point I'm trying to make. You don't have to believe me, but I'm telling you that my friend is a hyper-rational, highly intelligent, well informed individual. The fact that such a person could move from "I have never worried about being brown when moving around the USA" to "I will not visit anywhere outside of large metropolitan areas" - that's the change. You are free to argue that it is is irrational, or even "ridiculous" as you put it, but the point is that the state of American society pre-Trump left this individual feeling safe, and the same state post-Trump left them feeling anxious. If they were a person prone to irrational, emotive decisions or generally over-reacted to anything, then I would be inclined to dismiss the change in their sense of their own safety. If they had always worried about this aspect of life in the USA, I would be inclined to dismiss the change. But they are not and they did not, and so I take this change quite seriously.