Properly, pastiche is a method of training and not a legitimate mode of culture.
Pastiche is when the primary goal of creativity is to copy another: when a musician plays in the style of another, or when a writer writes in the style of another.
This is why impressionists are never first rate: they are not even trying to do their best, instead they are trying to be someone else.
Because if you are not even trying to do your best, there is zero change of being first rate in any way, shape or form.
But contemporary pastiche is even worse than this.
Pure pastiche is one step removed from attempting excellence, contemporary pastiche is two steps away because it tries to disguise its pastiche nature.
So we get the performer or creator first trying to emulate some other, then making some kind of changes in order that their impression will not be detected. Covering their traces. Muddying the waters.
So you might get a singer who is doing a pastiche of Elvis: at one remove this is an Elvis impressionist - who may be pleasant but cannot, of course, be first rate; but at two removes, we get the innumerable supposedly unique/ themselves singers who make changes such that their copying of Elvis cannot easily be detected.
This is even further from the first rate than a straight impressionist!
As a topical example Amy Winehouse is pure pastiche in a popular genre. It is hard to say exactly which American blues singer she has copied and changed - but clearly she is essentially a gifted impressionist masquerading as an original talent.
This kind of thing is absolutely rife in the high arts: as well as musical performance and composition it can be found in poetry, in novels, in movies... everywhere. Contemporary art is essentially a giant scam by gifted impressionist/ tricksters.
And it is undetected, because people just use creative art as lifestyle and mood simulators - so the real thing is no more necessary than it is necessary to have a real Monet on your wall instead of a poster.
Pastiche art merely serves to remind you of the real things - and for many people that is enough.
(Which is just as well, because nothing better is available, in most cases. The world of professional and collectable fine art, for example, is nothing but pastiche - or worse.)
Indeed pastiche extends far beyond art - since it could equally well be argued that we have almost exclusively pastiche science, technology, politics, policing, law and spirituality. Old stuff copied, tweaked and re-combined to appear original.
By contrast, when gifted people are really trying to do their best work, using whatever genre and technique they have learned and inherited, you can get first rate work in the most hackneyed of forms.
An example was Scott Joplin, whose straining at the boundaries of his inherited tradition of commercial tin pan alley pop, led to some of the most perfect and delightful miniatures of post-romantic classical pianism: utterly sincere, and with no trace of pastiche.