I don't actually think modern pop music is mediocre. Or certainly it's not any more or any less mediocre than any other era or genre of music.
I admit that I don't listen to a huge amount of it but although what I do hear can be a bit samey, that can be said of any genre of music can't it? And at the end of the day it is of course all highly subjective. I for one would rather listen to Little Mix than any number of leather trousered, shirt opened, microphone swinging, Les Paul playing, classic rock bands.
So as I don't actually agree with the premise of the OP I realise now I don't really have anything to contribute so I will back away hoping nobody noticed me come in