For many people the nomination a Conservative Supreme court justice was the only reason that they voted for Trump, so in that sense it is an accomplishment in that he satisfied these voters. Since this is the only concrete action that he has taken that falls into this category, he has to use it as his main selling point.
But clearly this wasn’t something that took any particular skill on his part, since all it required was his asking prominent republicans for a list of names and picking the one at the top of the list. Which anyone with an IQ over 80 could do. As to his crowing that no other president had done this in a hundred years, that is pure farce, and speaks to the fact that most presidents don’t have a vacancy that their predecessor left hanging open until they took office. Effectively demonstrating the exceptional nature of the Republicans stonewalling Obama’s nomination.