It's put-on for the "Nanny" character. If you heard her speak in her "real" voice you wouldn't know who it was. geeez
Now I am not familiar with everything she has done nor have I watched any interviews, etc. so maybe she can supress this voice. Or maybe the interviews are “acting”.
Jim Varney (Ernest P. Worrell) was a Shakespearean trained actor. But that isn't how most people would picture him.