I enjoyed this Smithsonian blog with scientific evidence that popular music has gotten worse, but actually I think the report it quotes says something different: pop music has become louder and dumber.
A report from the Artificial Intelligence Research Institute of the Spanish National Research Council in Barcelona dissected popular music between 1955 to the present via three categories: timbral (tone quality), pitch and loudness. It found timbral variety and pitch content have both decreased while loudness has jumped. Basically, for a long time pop music has become more homogenous in sound (possibly because a few dudes are copying themselves over and over to produce the hits) and simpler on a melodic level, while pumping up the volume – probably in an attempt to cover the deficiencies in the above two departments.
This corroborates my theory that music means less to contemporary listeners on an aesthetic level. Since audio technology has basically enabled all of us to have soundtracks, we treat it like background music and don’t mind getting served more of the same. Of course, I just posited that the music criticism/promotion machine encourages a strict genre narrative regimen, so the lack of engagement with samey-samey music may not be the listener’s fault. In addition, I suggested elsewhere that popular music has become dumber because the people trying to sell it to us don’t have high opinions of our intelligence.