John Irons: “What is Luffy’s mother like? Was she beautiful? What happened to her? Did she die when Luffy was a baby?”
Oda: “I think she’s alive. I’m still thinking hard about this. [Laughs] But if she does appear in the story, then she’ll be a very tough-looking woman. And strict. There’s no way that she’s a beautiful mother. She’s got this typical middle-aged woman’s permed hair. Speaking about families, I did a backstory for Nico Robin and her family.
That was actually the first time for me to draw a mother in a character’s memory. Up until that point, I’d never drawn a blood relation. In Chopper’s case, he’s called a son even though he’s not blood-related. So I’ve put in a message there saying that you can still call someone who’s not related by blood your family.” (American Shonen Jump, December 2009)
Looking at this interview, we can see how the description of Luffy’s mother fits perfect with Dadan 😀
Dadan was introduced in April 2010, and maybe Oda decided to use the description for Luffy’s mother someone who played Luffy’s mother.
I mean Dadan is a very tough looking and strict person, She is definetly not a beautiful mother and she has got this typical middle-aged womans permed hair.