That's a really good point. Not all design elements look right on all bikes. For example a Manx Norton tank looks fabulous on a Norton but would look out of place on say and RZ350.
I agree that crossovers on a aircooled just look wrong and it's probably because we are used to seeing them on more modern bikes but they look out of place on a square old RD. It's partly what we are used to seeing. The brain tries to match things to patterns we are familiar with and if it doesn't find something to match with we consider that it looks "wrong".
That comes from our ancient ancestors who looked out of the cave and when they saw something moving they had to quickly decide if it was something they could eat or something that would eat them. It's all about pattern recognition.
It's also why some things look, smell or taste horrible but we still like them because they feel familiar.