Libertarianism is just not a popular ideology, it's the ideology of the 1% and that's how much it can expect to get in any given election.
In most years, yes. This year....the major party candidates are so unpopular (especially Trump, at least right now) that I could see him getting a few %
if he gets sufficient media attention to make the voters aware of his existence. He managed to get 1% in 2012, even when the vast majority of voters had never heard of him, nor were aware he was running. If his current level of media coverage sustains or grows until November, then I see no reason why he couldn't match Nader's #s from 2000. But 20%? That's a pipe dream.
