I don't think that is the reason people are say that Trumo is not a legit president. The constitution clearly states that the President is selected by the vote of the electoral college. I don't know why that is, or if the reason it is still exists, but that is what it is. The reason I have heard for the "not legit" is the facts regarding the involvement of the Russian influence on the election and the possible collusion of the Trump campaign with the Russians. We do not know the details, and may never know. Comey did not bother to tells us about this, yet he inserted himself into the election 11 days before election day and probably swung the results to Trump.
The popular vote should call attention to another major problem. The Dems collected almost 3 million more votes than the Repubs, yet the Repubs have more than a 40 seat majority in the House. The house is supposed to represent the population as a whole. It does not. How can we say we have a democracy, which is one person one vote. The president and the Senate are not supposed to represent the people as whole, the House is supposed to. Look at our great state. We have 36 house seats, 25 are held by Repubs and 11 by Dems. That is 69% are Repub while only 31 are Dem, that is not representative of the voter who are allowed to participate in elections. This may be OK with you if you benefit from it, Can you change it back to a Democratic system of government when it swings to the other side? The attempt is to keep it the way it is by denying millions of people the right to vote, using voter suppression based on bogus claims of illegal voting. There is absolutely no evidence of illegal voting other than a few isolated cases.