The Nietzschean position is that morals,like philosophy,art,politics-culture in general-are the result of human creativity.They do not have an absolute Truth correspondence to a posited higher order [a higher order of 'essence' for example-whatever that might be].
Morality is not a Law dictated to man from any supposed outside force which some say governs the universe.

TO NIETZSCHE MORALITY IS AN INTERPRETATION BY MAN OF THE FORCES WHICH SEEM TO HIM TO GOVERN LIFE.

It is a 'common' mistake to think then that one's own interpretation is actually part of some immutable supernatural realm of 'essence'.In fact this is no more than superstition.

From there one can rightly say that morality is a matter of opinion.