It's ridiculous. He can apologise for himself, if he wants to. He certainly can't speak for every Serb in Serbia, especially when most of them had nothing to do with this.

In a representative democracy, a person is elected by the people to lead and represent them (the President). They are the only person who could make a national apology. An average Serbian making the same apology means nothing because that person wasn't elected by the Serbian people to represent them, whereas Nikolić was.