While it doesn’t make me happy to say so, arguing about whether Trump has a “mandate” is spin. Can’t be resolved factually. As rules-bound conservatives point out, it’s a concept with no legal weight, and rules-bound conservatives will dispute its ethical weight, since it’s not defined by the rules, “we’re a republic not a democracy,” etc. QE (to some people) D.
Sadly, our current political system doesn’t penalize elected officials who win with a minority - or, as in this case, a smaller plurality than another candidate. (A reason for small-d democrats to think about how to kill the Electoral College, and fight for some form of proportional representation in the legislature, IMO. Regardless of partisan identity.)
On the other hand, an elected official who wins in such a manner should, if they respect small-d democratic ideas like “one person, one vote” and “representatives of the people should represent the people,” keep the votes of the people in mind.
Trump almost certainly won’t - as GWB didn’t - and that’s his prerogative. Arguing about how he should respect his lack of popular mandate is likely futile in terms of affecting his behavior. (Though with an insecure narcissist, appeals to popular opinion might work? Naw, he’s got plenty of Twitter followers and rally crowds. Those critics probably voted illegally. Never mind.)
But I think it is both useful and ethically correct to use such arguments as partial evidence in explaining why Trump’s opponents are opposing him and his actions.
If you want a more small-d democratic government, then shout from the rooftops about how Trump was elected undemocratically! When Trump proposes something (or does something) you think is not just wrong but also unpopular, use his demonstrated unpopularity - the popular vote in 2016 - to bolster your argument! Don’t feel ashamed. You are correct. Not only did most Americans vote for someone else, more Americans voted for Clinton than for him.
But don’t expect to convince him, his backers, or the rules-bound conservatives who use “dem’s da rules, sorry!” as an argument to defend his right to fuck things up. They don’t care about “mandates.”