The research on this topic that I'm aware of fails to account for the fact that the top 40/100 lists are less representative of what people are actually listening to than they used to be. If Drake can drop an album and have every song on it chart on the Hot 100 for a week or two, that's going to influence the analysis. That simply wasn't possible before music downloads/streaming. You can see the impact on the chart records -- artists from the past decade dominate.
ETA: And "worse" in these studies tends to be defined in terms of measurable qualities where contemporary pop music most differs from "classical" music.
https://en.wikipedia.org/wiki/List_of_Billboard_Hot_100_char...
ETA: And "worse" in these studies tends to be defined in terms of measurable qualities where contemporary pop music most differs from "classical" music.