A progressive rips Johnson for the regressive nature of his tax plan. He's criticizing from a progressive (Sandersnista) perspective, but the tax plan is not libertarian either.
It's disappointing that the Libertarian candidate would give so much weight to this sales tax idea (the "fair tax"). At best, it is a massive distraction from real libertarian issues. It can't even be justified as the price of winning financial support from the Koch brothers. I can only assume that Johnson, like many conservatives, has a savings fetish and thinks that "libertarianism" is synonymous with capitalism (i.e. rules that favor the owners of capital). But at the end of the day, Johnson is proposing to expend massive political capital just to play an accounting game.
Johnson is also banking on the fact that income tax enforcement is particularly intrusive, though I doubt a massive sales tax would be much better. On a more substantial level, I believe that progressive taxation (higher rates for the wealthy) is more libertarian than other tax systems that raise the same revenue. This is because a person with more money (all else being equal) faces fewer constraints from losing any given amount of money. This is true even for losing the same percentage of their income -- it is worse to take 10% from a person with a thousand dollars than to take 10% from a person with a million dollars, even though the later involves 10x as much money. With this reasoning, it's clear that reducing taxes on the poor should be the first financial priority of libertarians. Lest we are worried about being unfair to the rich, they are fully capable of advocating for themselves in government.