Mayor Michael Bloomberg's approval rating may have dipped but New York City voters say he is a better mayor than Rudolph Giuliani and would make a better president than his predecessor, according to a poll released Wednesday. . .However, voters still said that Mr. Bloomberg is a better mayor than Mr. Giuliani by a 46%-to-16% margin, and that he would make a better president than his fellow Republican by a 46%-to-31% margin.