Oh that's very sad. My mother loved MTMS & Rhoda. I got her the dvd's of both. Marge Simpson is in that, the actress that does her voice i mean. Her voice isn't really that different from the character.
I think you're referring to Julie Kavner who actually does the voice of Marge Simpson. She also played Rhonda's sister, Brenda in 'Rhonda'.
Indeed Valerie does have a guest spot in one of the 'The Simpsons' episodes this year.