Yes, you’re right. Djokovic has greatly underachieved while Nadal has massively and wildly overachieved at that event.
"Underachieved" and "overachieved" are subjective terms, and in speaking of the Big 3, in particular, tend to be either weaponized, or used to express bitterness. We often say that talented players like Safin and Nalbandian, and, say, Dimitrov, underachieved in their careers. That's not a controversial statement. They never accomplished what we hoped they would, or expected from them, based on their talents. Rightly or wrongly, we all accept that. When players have 20, 19, 17 Majors, however, it starts to seem petulant. Federer fans have long been complaining that Roger "underachieved" at the USO, having "merely" won 5 times; and of late they've taken to complaining that he has "underachieved" at Wimbledon, even though he holds the Open record number of titles at 8. And now you're complaining that Djokovic has underachieved at the US Open, even with 3 titles, to date. Three Majors alone being a hall-of-fame career.
"Overachieved" is the one that's more complicated arguments in sports. Complicated, I would say, because you actually achieved the thing. David Ferrer makes a good example, as does Tomas Berdych, I think. While neither was especially possessed of a great game, they were very solid. Berdych had the use of his height and his serve, Ferrer of his grit and being a great returner. They reached 3 and 4 in the world, respectively, and the highlights for each is one Major final, and a title at Paris Bercy. Did they overachieve? Well, they achieved what they did. As to an elite player, a Big 3, like Nadal: can it be said that he has overachieved, when he has achieved so much, and his ceiling is so high? I don't think so. This is where the bitterness comes in. And the subjectivity. If a player were actually "overachieving," there is plenty of opportunity for that to be exposed, particularly at a Major, where they play 7 matches, and particularly on HC, I would say, where most players excel. If you won the tournament, you didn't "overachieve." You just won it.