This is a common misunderstanding. The title of "best fans in the country" specifically relates to the way that Nebraska fans act while at a Nebraska football game. It does not, and in my opinion was never intended to, apply to all Nebraska fans at all times.
As people, we aren't any different than fans of other teams. We have nice people, boorish people, stupid people, belligerent drunks, asshats, etc....just like every other team.
But for many decades there has been a collective ethos of the fans attending a game at Memorial Stadium, that they would act with a certain amount of class. And if you didn't, there was probably a blue haired old lady sitting behind you ready to shout you down or call for security.
That is what has led to the title of best fans, and based on the experiences of visiting teams and their fan bases, it has been warranted.