Here's an entry for Sesame Street from the book "This Year in Television, 2009: A Catalog of New and Continuing Series."
Among those listed in the "Principal Cast" are Julianne Buescher as "Sherry Netherlands" (sic), Tyler James Williams, and Bill McCutcheon as... "Uncle Dudley." I'd hardly consider Tyler James Williams part of the principal cast.
The Muppet performers are credited as "Muppet Voices," of which Julianne isn't listed (she's grouped with the human cast). And Caroll Spinney's last name is spelled with two "R's" (okay, okay, how many of us haven't gotten that one wrong?)
Another TV encyclopedia, by the same author, lists Bill Cosby, Marilyn Sokol (more specifically as her "Aunt May" character from the "Big Bird Goes to Camp" shows), Zoe Hall (who?), and voice actress Heather North (as a character called "Ellie Jane," whom I'm assuming was an animated character) as part of the main cast.
Elsewhere, it lists Miss Piggy as one of the Muppet characters featured on the show.
The entry for Muppet Babies lists Howie Mandel in the credits twice, and says that the series takes place before the Muppet became stars on Sesame Street.