On the one hand, legislatively speaking, a Republican house + senate + President gives a strong mandate, legally.
But losing the popular vote in a divided country decreases that mandate, not legally, but perhaps morally.
You don't want to make the Allende mistake. He got 36.6% of the vote vs two other candidates with 35.3% and 28.1% of the vote, and thought that they gave him a mandate for land grabs and massive forums.
I'd argue that Trump should try not to make massive sweeping changes like repealing Obamacare, building the wall, and so on, but respect the more modest mandate he has won. If he doesn't then things could get pretty ugly in the US. However, at the same time, he needs to try and respect the promises he's been elected to deliver, so there is a balance there.
If he's over promised vs what he can deliver, he could cite the popular vote for Hillary and the decided country as a reason for not keeping certain promises, if in any case he finds it difficult to push through certain things.