According to Google, music is “vocal or instrumental sounds (or both) combined in such a way as to produce beauty of form, harmony, and expression of emotion.” Step forward, modern “cool” rap. Brusque curse words for pointless, PG-13 subjects- so much “beauty of form, harmony, and expression of emotion!”
Music is auditory art and art is expressive of a story, message, or feeling. Come up with a creepy and really weird story or message, make a poem that overuses poetic license out of it (with a very protuberant beat), and you’ve got a “cool” rap song! But, who can say most aren’t catchy? Oh, how things evolve.
Its popularity is very plausible- no judgment to those who really like it! I am simply expressing my personal stereotypical-grandma-like opinion of the music type.