To my knowledge, LL Cool J doesn't play an instrument to start with, he's a rapper. The backing band however, were playing instruments. That was the purpose of having an episode of Unplugged dedicated to rap. I also regard the turntable as instrument, which is how it's used in rap and hip-hop.
What mainstream bands are you referring to?
There were also tons of teenybopper artists filling the charts back then, same as there are now.
I love Led Zeppelin, they're one of my very favorite bands but I don't put them (or any artist for that matter) on a pedestal.