He ran on the promise of building a wall. Pretty obvious that it should be built, if America still has a functioning democracy.
Regardless of whether it's stupid or not, a waste of money or not, how does it reflect on American democracy if even a president cannot cash on his promises? Where's the incentive to vote if nothing will be done? When even the elected leader of a country is without authority?
Like with ObamaCare, or pulling out of Iraq, you had to get it done, because it was a large part of why people voted for the guy. If it hadn't been done, it would have reflected on the government even worse than an unfinished package or a premature withdrawal of troops.
A half-assed wall, or fence, is still better than no wall. At the very least it will serve to reinforce some kind of trust to the people that voted.