I think a better example would be Cold Play. They routinely sell out stadiums but are truly awful and normal people hate them.
The hate that Nickeback gets is of course valid but pales in comparison to the hate Cold Play so and should get.
Makes you wonder about Avril Levine.
I mean yeah...a niche following is still a following none the less.
Perhaps a better comparison would be some of the conventions they have that sell out.
For instance, ComicCon and Wizard World. My cousin goes to those, and they sell out and sell fast when tickets hit the market (he and his friends get up early and refresh the webpage like in that scene in Big Bang Theory),
...but in reality, it's likely way under 5% of the population that are die-hard fans of that sort of stuff.
I'd suggest that it's largely the same with political figures. Given that we're a country of 300 million people, if you go to an area that's particularly friendly to whatever it is that's getting pitched, it's not surprising that someone can muster up a big turnout.
Where the two situations differ is that ComicCon knows they're a niche sort of thing, they see it for what it is. Or like a concert, the goal is to pick a "friendly" area where they know they have a lot of fans, and run a show.
In politics, strategists seem to have some tunnel vision and assume that these kinds of big regional turnouts are somehow emblematic of a wider popularity.
It'd be like if a Hip-Hop artist sold out a venue in Brooklyn, and used that successful show to assume "that must mean we'd be really popular if we went to a place in rural Iowa"
If Trump wants to really gauge how popular he is (or if the RNC wants to get a read on it), they shouldn't be assuming that being able to draw a big crowd quickly in Pickens, SC (a county that's 78% republican, bordered by 3 other counties that are >65% republican) is somehow evidence of widespread appeal.
It's the same mistake I've highlighted that the democrats make from time to time...assuming that because a particular candidate or policy is extremely popular in NYC, LA, or San Fran...that'll it'll somehow play in middle America.
One notable example is that I've critiqued democrats on their affinity for "Manchin-Bashing". They assume because he doesn't meet the coastal standards for "real progressive/liberal", that "we should primary him and get him outta there, we can do better", to which I tell them, "In West Virginia, no you can't".
Republicans make the same mistake in the New England region. What's popular in South Carolina isn't always popular in Massachusetts, Maryland, or New Hampshire...which is why you see a lot of instances of people in those states voting for moderate republicans for governor, but when it comes to general elections and presidential candidates pandering to southern politics, those states go blue.