Question

Why has quality of pop music gone so bad in the last decade?

Answer 1

1. Pop music has become slower -- in tempo -- in recent years and also "sadder" and less "fun" to listen to. 
2. Pop music has become melodically less complex, using fewer chord changes, and pop recordings are mastered to sound consistently louder (and therefore less dynamic) at a rate of around one decibel every eight years. 
3. There has been a significant increase in the use of the first-person word "I" in pop song lyrics, and a decline in words that emphasize society or community. Lyrics also contain more words that can be associated with anger or anti-social sentiments.
hope it helps you

Answer 2

Kind of a strange question. Decline in the quality of popular music since...1650? 1970? 1995? Whatever the time period you're thinking of, the answer is likely the same: there has been no meaningful decline in the overall quality of popular music. Rather, there has been a steady evolution in production and distribution technology that has changed the way we consume music.

A couple of examples from the last 15 years. The two major developments in the popular music world over that time period are:

The fall of the mass-distributed "physical" music product (i.e. record, tape, CD), and the rise of the internet as the dominant distribution network for recorded music.
The disambiguation of the musical artist and the musical personality.

Relative to #1: as Napster and later iTunes (and MySpace!) facilitated rapid discovery of long-tail music selection, both music producers AND consumers arranged themselves in much smaller niches. So, indie rock fans no longer listen to mainstream rock radio to discover new music; they don't even need to interact with the greater indie music community. They can just read Pitchfork and have extremely relevant content pushed to them directly. As a consequence of this microtargeting, the remaining mainstream music outlets (MTV, pop radio, etc.) have had to move even further to the center, since any even slightly fringe interest is better served by more personalized content sources. This is why variety on the radio has decreased and our biggest musical institution, American Idol, caters to middle-aged Middle-Americans. The only way they can compete with personalized sources of music is by casting the widest net possible.

Relative to #2: this is not a new phenomenon; producers have been writing music for performers for as long as there's been "pop." We've just taken it to its logical conclusion. The Monkees were a joke band, but they had to play their own instruments on stage. Ricky Martin may not have been a musical genius, but he was ostensibly still singing on his albums. But with the advent and rise to prominence of Autotune, the name on the record now has more or less nothing to do with the artist responsible for the content. A KeSha album (not necessarily the best example) is really a Dr. Luke album with guest talking (pitch-shifted into "singing") by KeSha. This has increased the perception of the "musicians" we see on MTV as "talentless hacks." They are indeed talentless hacks, but the music is no better or worse; it's just created by someone else, and the someone elses are as great as they've ever been.

Summary: more good music is easier to reach than ever, but "pop" has converged towards the mean and the true musicians may be less visible.