Johnson is actually helping Trump more than hurting. Johnson is an open borders more lib than libertarian.
Johnson and Weld, the Libertarian candidates for President and Vice President are not libertarian in the Murray Rothbard-Ron Paul sense. They are basically fiscally responsible, low tax liberals with an isolationist streak. As such, they appeal to RINOs and could hurt Trump in the precincts Kasich and Rubio carried. They will receive a level of publicity that no third party candidate since Ross Perot received.
According to polls of his supporters, Johnson draws roughly as many people who voted for Obama in the last election as he does people who voted for Romney. Having Johnson in the race probably won't affect Trump or Hillary's margins one way or the other.