There are several ways that postmodernism has been defined. One of the ways that it has been defined is as troubling to me as it is to many PoMo detractors. That being so-called "moral relativism". When human beings can no longer tell right from wrong we cease being human beings. I think that view is a corruption of postmodernism. Just because everyone has their own version of reality does not mean that each view is of equal value. If someone's reality is that it is OK for people to suffer needlessly – well my reality is that that person is wrong and should be stopped. But… for me postmodernism is mostly a descriptive term for the era that we live in.
Here is how I would describe it:
– Pre-modernism was defined by powerful external forces like the church and the monarchy. "Reality" was whatever the chief or the shaman said was "real".
– Modernism was characterized by humanist rationality – the search for "universal" truth – think of Descartes, Voltaire, Spinoza, Darwin, etc.
– Postmodernism in my view isn't really "post" modernism at all, but a term used to describe late modernism.
It includes two broad, basic, concepts.
1) Eclecticism – there is no longer the need to find the "One Right Way", or the Universal Truth. Every person has their own version of what is real. ***This is sometimes misunderstood to mean that all versions are equally valid*** I think that this is what you (rightly) object to.
2) Baudrillard's "simulacrum" – the copy without an original. This can best be seen in American television. TV shows an idealized fictional reality which people then emulate and which television then reflects back to them again. Another way of thinking about this is as a hall of mirrors, an endless reflection of reflections.
Beyond those two key factors postmodern philosophy becomes very muddled and confused as it begins to get tied up in knots. It becomes completely useless as a basis for anything besides high-minded parlour games. I mean once people start using rational, logically constructed arguments to "prove" that logic and rationality don't matter, the whole exercise becomes ridiculous.
From an artists point of view postmodernism is really just something to have fun with. Artistic eclecticism opens the doors to all kinds of creativity. Video art, sound art, language poetry, etc. are all examples of postmodern artistic expression. Also, since art has always tried to reflect back an interpretation of the artists reality, the simulacrum is already "built-in" to an artists life and, I think, always has been.