I'm quite interested in hearing more comprehensive answers from a specialist, especially regarding how they viewed non-Mediterranean art, but a quickie answer:

They were unsurprisingly capable of recognizing unskilled art. There's a great story about the great painter Apelles delivering a burn to a lesser contemporary painter: "You didn't have the skill to paint Helen of Troy beautiful, so you have instead painted her rich [by portraying her as wearing lots of gold jewelry]." Pliny was [quite opinionated](http://www.masseiana.org/pliny.htm#BOOK XXXIV) on art quality. Older works were counted as "rude", and works that were overly fussy were described as "assiduity has destroyed all charm."

The Greeks and Romans also had fashions, just as we do today. One example I can think of is vase painting. The Greeks used to do black-figure / red-background vase painting, but when someone figured out new firing technique, red-figure / black-background vase painting quickly took over and dominated within a century, if not within decades. I would guess, though I don't have a source, that black-figure vases would be seen as outmoded: not bad, just not the thing you'd want in your house as a fashionable person.

This metope is somewhat well known for being, well, just not particularly good. The centaur's legs are wompy jogged and the problem with the height of the figure is solved with the rather inelegant method of transporting its head into the middle of its chest. Compare this similar scene, handled much better.

It is actually a pretty good demonstration that the Parthenon was handled by multiple artisans.