So how about some simple, unambiguous fact that should submit to easy verification?
As of late last year, about 42% of Republicans polled disagreed with the assertion that Obama was born in the US, while only 27% of Republicans agreed [1]. Plus the fact that our current sitting president was instrumental in formulating and then popularizing this lie.
I submit this to you as a good example of 'know-nothingism' -- if there's some piece of information that challenges your world view, then you will do your damned best to know nothing about it.
[1] https://www.nbcnews.com/politics/2016-election/poll-persiste...
"Know-nothingism" derives from the Know Nothing party of the late 19th century that was known for its anti-immigration views. The Know Nothings originally held secret meetings and would tell people "I know nothing" when asked about it. So it was not originally a derisive term.
Then starting in the 20th century, nativist political sentiments (and this includes the "America First" mentality espoused by our current president) was called "know-nothingism" by liberal commentators.
So now it has morphed into a term of derision, however it still maintains its attachment to nativism, although anti-intellectualism is obviously a big part of the stereotype.
This is all to say that yes, know-nothingism is sort of a shorthand for Trumpism as far as I can see it, so it's kind of a truism to say that know-nothingism is prevalent on the right. As far as whether it's poisonous, or equally as poisonous as the cultural relativism of the left, I will demur.
Edit: runon and typo
There's the moral self-loathing (veering towards self-flagellation) of the 'social justice' class, the academic anti-truth of the relativists and the post-modernists, the equally poisonous know-nothingism of parts of the right.
Western civilization still lives in the long shadow cast by Europe tearing itself apart not just once, but twice in the span of a generation.