Has a presidential candidate taken a state with less than 30% of the vote? Wilson took Idaho with 32% in 1912. That's the smallest plurality I could find in the 20th century.
I realize this would require McMuffin and Aleppo both to do extremely well, which probably is not realistic.
I think that Johnson will lose some of his support to McMullin as the protest-voting Mormons find out about McMullin (a poll showed that something like 66% of Utahns don't know who McMullin is), so it would be hard to beat that record set by Wilson.
What he could beat (not sure though) is the lowest nationwide popular vote while still winning a state. I haven't looked into whether or not he could, I just had that thought yesterday.