You need to look at things in totality.
- won the popular vote by over 2M votes
- won by electoral college by 84 votes
That alone would be “a solid showing”.
But he also:
- carried the Republicans to maintain a majority in the House
- helped increased the margin of seats in the Senate to a majority
Many members of the Republican party had been against Trump. This election, most were pro-Trump. His popularity helped pull both houses.
And did this after the opposition tried every tactics possible to ruin him - a fake Russian dossier that was peddled unquestioningly by the media, a misdemeanor crime gets prosecuted as a felony in the hopes to bankrupt or disqualify him from running again.
Against those odds? That showing is remarkable. Oh and he was almost assassinated, twice.
Then add on top he increased Republican share in solid Blue states like NY and CA by double digits. He increased Republican votes with black and hispanic voters.
He basically increased every single voting group across the board.
In light of the challenges, the outcome is a clear showing that a significant percentage of the US voters support him and the people in his party that are aligned to him.
I'm not sure what else you'd need to show a clear mandate.