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.
Yeah, I mean the popular vote issue wouldn't get the attention it's getting at the moment if a more benign Republican had won.
I note that Hillary (a more authoritative voice on the matter than anyone on an obscure political forum since she, you know, is the one who suffered this fate) hasn't, as far as I'm aware, raised the issue since the election.