Wouldn't say Nishikori is a joke, he is just a Ferrer level player. But he could be even better.
Certainly though he's a cut below who Federer
beat in his pomp. So you can see why people would think it's pretty stupid to elevate Djokovic above Federer when he is susceptible to these kind of upsets in majors.
My feelings on the Djokovic - Federer GOAT discussion remind me of the Kobe Bryant - Michael Jordan comparison. Not accomplished enough yet to take the mantle, and might never end up becoming accomplished enough.