I'll have to look it up, since I haven't read it since I was in college (mumble) years ago, but I recall a folktale about Bertha Broadfoot (perhaps the title?) that was in the category of a "royal bride forced to switch places with her servant" tale. A note at the end said that she was supposedly the mother of (?) Charlemagne.
As for your real question, I always thought that the storytellers, musicians, etc. being blind or crippled was how they ended up being performers. In times gone by, it was something that they could do (assuming they had the knack for it) in spite of their physical condition. It goes on to this day; Stevie Wonder and Ray Charles might have become musicians if they could see, but there would have been more possibilities open to them as well.
For information on Aesop I'd suggest going to the library (or the library's website, I think this is online by subscription) and looking in Something About the Author. It's a standard reference book series for author biographies. I know they've got quite a bit on Aesop and the various versions of his life and death. I never looked up Mother Goose in SATA, so I don't know if you'll find anything on her, though. Good Luck.