In her story called Some Zombie Contingency Plans, Kelly Link makes a point that sometimes people say "this isn't art" when they are looking at something that could obviously serve no other purpose than to be art. I think what people usually mean by "this isn't art" is "this isn't good art" and, quite often, that it was a waste to produce. A waste of the raw materials that went into it, a waste of the alleged artist's time to produce it, a waste of space now that it's done, and a waste of the audience's time to be subjected to it now. And if something is waste, we have different words for that. Junk. Garbage. Rubbish. Landfill. Waste.
It sounds quite harsh to put it that way. If you have attempted to produce art, and have failed, then you have produced garbage. But if it isn't good at being art, then that's just what it is.
So what does it mean to be "good at being art"? That probably depends on whether you ask a postmodernist or not. If you ask the average person, they will tell you that a piece of art is good if it causes the audience to appreciate beauty, to contemplate some notion, or to feel happiness, awe, sadness, or some other "noble" emotion. If you ask an artist, they will probably tell you that their art is good if it conveys what they intended - it makes the audience feel what the artist wants. If you ask a postmodernist, all you will get is the argument that art is art if it makes you feel anything, regardless of what the artist intended, or even if there was no artistic intent. Art is art if it makes you feel a feeling.
Mokalus of Borg
PS - Even if that feeling is unpleasant.
PPS - As long as that is the artist's intention.