Why is Iron Man called Nini?

Because his name is Tonystark, and Nini is a nickname, calling him a friend of Iron Man Nini, and most of them are fans of Iron Man or actors of Iron Man.

Iron Man is a sci-fi adventure film produced by American Marvel Comic Film Studio. It is adapted from a series of cartoons of the same name, directed by jon favreau and starring Robert Downey Jr., gwyneth paltrow and Jeff Bridges. This film is also the first film in the "Marvel Movie Universe" series. The film was released in the United States on May 2, 2008.