Rap today is a mashup of old lyrics, over-engineered (and unnecessary) beats and a sad use of Autotune.
Mainstream rap, sure. But I don't think "rap" generally is worse. It's just that it's become popular enough for there to be a crass mainstream that didn't exist in the 80s.
It's common to bemoan the modern state of many things but we should recognize that while the mean quality may have dropped, the median probably hasn't. There's a lot of great rap still out there - it's just not the common definition anymore. (It's like bemoaning the commercialization of blogs or the low quality celeb accounts on Twitter - the good stuff still exists on both.)