I don't think there is anything wrong with changing your diet to improve performance. Djokovic eats a really clean diet, which I am sure is good for him for a bunch of reasons.

I do think it is a bit dangerous to start blurring scientific and unscientific reasons for doing things. If you read Djokovic's diet book, he does this a lot. He mimimises added sugar to avoid spiking his GI (sensible) but eats 'mindfully' so that his body 'digests better' (wtf).

When you start making up your own rules about why things work, it becomes easy to make unscientific logical leaps (e.g. my diet works because I don't put artificial stuff in my body, a vaccine is artificial therefore I shouldn't use it).