Mariah is ranked #79 on that poll while Janis "Caterwaul" Joplin is #28.
I also take issue with the fact that there are virtually no country singers represented by that list. How can you have a list of the greatest singers in music without including country artists? The country genre has hosts of amazing singers. The females especially possess some of the greatest pipes in modern music in terms of range and power. Martina McBride, Faith Hill, Leann Rimes, Trisha Yearwood, Carrie Underwood, Jennifer Nettles - all are women with absolutely jaw-dropping multi-octave vocals and they are criminally ignored by that list.
So yeah, I hardly think this RollingStone list is definitive.