As much as we might all love pop music, Eliot uses it as an example of how crummy Western culture has gotten. Pop music symbolizes how mass culture tends to take objects of very important social value and utterly ruin them (like Dan Brown using the great works of Leonardo da Vinci to write a bestselling thriller—Eliot would not be down with that). Eliot is really, really not cool with art that's simply popular, because he believes that great art can sometimes be over people's heads, and that the majority of people don't have the good sense to appreciate it. So they just run back to their iTunes Top 10 and download whatever's catchy.
Lines 128-130: In these lines, Eliot quotes lines from a popular song (from his time) called "The Shakespearean Rag." The lines "It's so elegant / So intelligent" are said with total sarcasm, since stupid people have basically taken something that's actually intelligent (Shakespeare) and turned it into a song that drunk people like to sing. The word "rag" is especially suitable for this type of criticism, since it refers both to a popular form of song and also a cloth that wipes up filth.
Lines 199-201: In his notes, Eliot admits to lifting these lines from a ballad song he heard from Sydney, Australia. In reality, this version of the song is actually one of the cleaner versions that Australian troops sang during World War I. Again, popular music tends to insert itself in Eliot's poem as an interruption, a type of modern noise that's always drowning out meaningful thought with nonsense.
Lines 266-291: Eliot takes a song he would have respected—from a Wagner opera—and fills it with his own lyrics to show how the great accomplishments of the past are pulled down into the mud and filth of the modern world, which is defined mostly by unthinking mediocrity. Everything is so horrifically average in this modern world, and the forms of popular music convey this sad fact better than anything else.