This is a common line used by leftists who are angry that Trump won and are looking for someone to blame. Third party voters make an easy target, and the left has long felt entitled to the support of third-party voters.
But this entitlement assumes that third party voters would otherwise have voted for Clinton, which is a pretty strong assumption that also doesn't really hold up against the polling data from late in the election. Johnson took more than half of the third-party vote, and had he not been running, most Johnson voters would either have voted Trump or not voted at all.
Trump didn't win because of the few voters who voted third party. He won because of the 63 million people who voted for him. If you want to blame someone for Trump's victory, blame them, not the 7 million who chose not to vote for Trump.
