No, the look doesn't have anything to do with performance; however, once the look changed, performance seemed to take a huge hit.
I think the performance hit had very little to do with the change in look, and more to do with the lack of focus on software stability, performance, and QA.
The same could be said about OS X. After it became free, I think there was a general decline in software stability and performance.
IMO, the most stable OS X was 10.6, which was the last one to cost $. After the stability and footprint issues of 10.5, Apple focus purely on the performance and stability of 10.6. It didn't have a boat load of new features, but it was a rock solid OS.
But, since then new versions became free, and we got Lion.
10.8 was pretty stable, and probably the second best since 10.6, but 10.9 sucked.
10.11 was probably the third best since 10.6, but again, there was a decline with 10.12 and 10.13.
While iOS 12 will probably not bring back the look of iOS 6, I don't think that looks was the reason for lack of stability for every iOS since then. The good news is that the rumors say that Apple will focus on stability and performance, similar to MacOS 10.6 after 10.5.