I don't know about dumber, but certainly louder.

http://www.cbc.ca/books/2012/08/is-pop-music-dumber-and-louder-than-ever.html

"A new study by Spanish researchers suggests that's the case. According to their findings, after analyzing 55 years of pop music history, modern pop is less musically complex and engineered at higher volumes. "

The latter part is definitely true. Go here - http://www.dr.loudness-war.info/, and enter in your favorite Album or Artist to see just how good or bad the dynamic range on the songs are. And yes you can compare original releases with 're-releases' or 'remasters' to see just how louder things have gotten.

I used to think record companies did it to make their product more competitive.  And on some level I think there's a connection with this and both the decline of CD sales, and the increase in online music sales.

But I wonder if there might be another reason. Perhaps the record companies were so afraid of copying, that they deliberately made a shitty product so that no one would do it.