Quoted from cottonm4:
Marilyn was the niece who was visiting and the only one "not normal". But to my understanding, Marilyn was there just as much as the other cast members. Correct? Yes? No?
Marilyn is in the Munsters home from the start. It was never said that the was "visiting," nor was it ever made clear why she wasn't living with her parents, who were said to be in Transylvania.
It's stated that Marilyn is attending Westbury College in Mockingbird Heights. Maybe the school is amazeballs, and that's why she made the trip from Transylvania.
The only other thing we're told is that Marilyn is Lily's niece, which makes you wonder why her name is Marilyn Munster. When the series was being developed, her name was Marilyn Mundane, which makes more sense.
Quoted from Jackalwere:
Sculpting another character adds additional cost not only to time and materials, but also to licensing because you've got to pay Pat Priest for her likeness - if she consents.
Let's not forget there were two Marilyns, Beverly Owen being the first (Eps 1-13), so which one are you going to sculpt? Tweeterhead, whose sculpts were used by Christopher Franchi for the pin, had originally planned to have interchangeable heads for Marilyn, but that idea ultimately met with the chopping block.