Most vocalists are mediocre pretty much by definition compared to the best.
Many mediocre, and even technically poor vocalists still have the "it" factor to be a "star" and over the years typically get set up with the the most talented musicians available to make a good product.
Bob Dylan demonstrates one need not be even a mediocre vocalist technically or even be overly physically attractive to connect and be a star. I would rather hear Dylan sing any tune he is committed to than than most very good vocalists who may just be going through the motions.
The example of a mediocre vocalist catapulted to stardom that comes to mind is Mick Jagger. I love the Stones but much prefer most studio versions of their stuff to most any live performance mostly due to Mick’s mangling of the vocals in comparison. Just me.
