I don't know why I'm even bothering with this, but here goes nothing I guess.
First of all, aesthetics is subjective, so the title and premise of the thread make no sense to me, as much as I share your distaste for mirror matches (unless it's a mirror between two players who base their own style on variety over consistency, which is pretty much what is missing).
Second, you can't accuse the kids of "imitating Djokovic". Nowadays it is not realistic to think someone old enough to develop a taste will turn on the TV, see one player or the other at work, say "I want to be like that when I grow up!" and actually make it to professional tennis. It's not like there is no example against this (though the only relatively recent one that comes to my mind is Estrella-Burgos); but when the vast majority of would-be pros start playing, they're a white canvas for the coach to paint on, no matter how much they say "Player X inspired me when I was little" once they start getting interviewed. And even though I disagree with the approach (in my personal opinion, it is better to develop a style that disrupts that of the majority than just copy the tried-and-true and hope your disciple simply ends up better than thousands of others at doing the same things), I cannot blame the majority of coaches for walking the path of less resistance, since the alternative I proposed also has several flaws that carry much greater risk for their career than simply developing players who look like 90% of their opponents and happen to not be the best among them.
Third, to accuse Djokovic of "ruining the aesthetics (or anything) of tennis" because of his playing style, said playing style should be original; and with all due respect, that is simply not the case. Leaving alone the fact that his playing style is a polished version of Agassi's, which in turn is a polished version of Connors', which in turn was already played by the vast majority of female professionals when he was a kid, the majority of male players already had that style or something close to it in 2011 when Djokovic exploded and first got to #1, and it was already very common in 2008 when he won his first slam as well. If Djokovic didn't exist, someone else would just have taken his place and at best (or worst, depending on one's point of view), the whole conversation would be shifted by a few years, and that is only in the assumption that neither Murray nor anyone else would be able to step up and interfere with the Nadal-Federer duopoly.
Long story short, there is always going to be one way of doing things that gets overall better results, and with that being the case, the choice is to either imitate and/or try to improve upon that way, or to try and develop a counter. The first approach leads to many people looking like they're copying each other, the second requires a lot of trial-and-error before finding success and as such, even in the unlikely scenario where 50% of the coaches opt for said approach when teaching to little kids, there will always be a minority of "different" players until someone makes success in a consistent and convincing enough way to set the new status quo, leading to a reset of the "everyone does it that way" situation after a transational period where we can observe some variety, as was the case for tennis in the 90s and early 2000s.
