No I don't think anything "good" requires an art defining or describing a period of time. More often good things transcend the period of time they were created in. To the extent that they do reflect a period of time it's because other, lesser artists followed their lead, over-saturating the art with a particular style for a period of time.

A great artist may define or describe a period of time incidentally, but you can be good without inspiring copycats, even if copycats are generally a sign of something being good.

tldr The cause of being good isn't that it defined/described a time. That's the effect if anything.