Like it or not, Superman/Clark Kent is obviously patriotic for his home country, America. He was raised in America and chooses to continue to live in America as an adult. Superman has also used his superpowers to help other countries as well, as he does in Superman: Peace on Earth (1999), by Paul Dini and Alex Ross, for example, which is reprinted in The World's Greatest Super-Heroes (2005).
In Superman's contemporary days, he has come into conflict with aspects of the American Government. Superman was at the White House discussing and debating with President Reagan in Legends (1986-1987) by John Ostrander and Len Wein, Superman was completely opposed to Luthor's Presidency (Superman: Lex 2000 (January, 2001) by Jeph Loeb to Superman/Batman #6 (March, 2004) by Jeph Loeb). Superman is not a nationalist zombie. Superman does use his abilities to help people in other countries. Superman never proclaimed that being a good person is something America came up with, and Superman has never tried to claim that only America has ownership of such values.
Superman being granted Citizenship by the United Nations has not been contradicted by any Superman stories. Which means that the United Nations Citizenship apparently is still intact. Even events from as far back as Action Comics #1, happened. In Superman: The Man of Steel #16 (1992) "Crisis at Hand: Hard Knocks", by Louise Simonson, Superman got involved with a domestic abuse case, and it established that the domestic abuse case from Action Comics #1 (1938) "The Coming of Superman", by Jerry Siegel, still happened, as Superman remembers it and a flashback is shown. What's been conformed not in continuity is this David Goyer story in which he has Superman proclaim that he will be renouncing his U.S. Citizenship.