I grew up and frequently jammed with Bob ( Zimmerman ) Dylan in HS circa '56-59 (I'm 2 months older) but I would probably never qualify him as a singer, if asked.
Agreed. Though, I don't remember him singing jingles either.

Also, the Everly Brothers were turned down by every studio in Nashville before 'making it'. They would play in the alley behind Ryman Auditorium hoping to be heard.
There is a difference though. They weren't turned down for not being able to sing. They had a sound the record companies weren't looking for at the time. I think we can agree they had a sense of pitch and rhythm and some pretty amazing talent.
What I'm referring to are people that don't sing in key...I don't mean as in a creative choice either. They also may not have much of a sense of rhythm.