He is not legitimate in any way. He didn't win the popular vote, and he was only elected because the Russians and the FBI teamed up to deal a death-blow to Clinton, not to mention the fact that his election sent the KKK celebrating in the streets.
In reality, you can't objectively prove that Russia's interference caused Trump to win the election. That the KKK celebrated his victory also doesn't somehow deligitimize his election, despite it being disgusting. And while it's true he lost the popular vote by nearly 3 million votes, he did win the electoral vote without any substantial indications of vote rigging. In America, whether we like it or not, the electoral college determines our President - not the popular vote.