I can't decide if Janis Joplin's singing is pathetic and clownish or if we still haven't yet appreciated the heights of her unique virtuosity.
I think her real problem was lack of great material. Robert Plant might have been unbearably annoying if he didn't have the benefit of the genius of Jimmy Page, and if the sound of Led Zeppelin hadn't been such a perfect-fit playground for Plant's Witch-King-of-Angmar meets Welsh triple harp voice.
Joplin had, perhaps, "Me and Bobby McGee" and "Ball and Chain" (maybe "Turtle Blues"), but that's it. Imagine her in the 1920s and 30s when the blues were rawer and wilder. Why she didn't cut more traditional, old-timey blues records instead of the weak sap, early '70s idea of "blues" she did record can probably be chalked up to everybody doing too many drugs back then.
It's so easy to blame hippies.