I would rather that her mindset came only from her situation, then from comments that the uncle might have made.
She is a young child, and thus self-centered. She only has a mother, no father. So to her, the mother is the most important thing/person in the world. She could not imagine a life without her mother, so she can't image others not having one.
Maybe she has been question about her father, and justified herself with the idea that a father is unimportant. Maybe even heard someone say that it's weird that she doesn't have a father, and now she is using that same line on others.
I mean, it also makes sense that she got that mindset from her uncle, after all, kids at her age specially, are very impressionable and would repeat things that adults in their life said or did, you know, monkey see monkey do.
So, I would say, it's a combination of both hearing it from her uncle, and also her situation.
Your theory makes sense but I think it's wrong bc she'd never questioned Hiro's parents before. Just like she has her mother and her uncle, Hiro has 2 dads and their friend has a mom and a dad, so to her all families being different was normal until she heard an adult say otherwise. This goes to show that prejudice isn't something we're born with, it's what we're taught







I know there is a manga about one of the cooks and the girl that is serving at the restaurant. But I don't remember the other characters, are they from a diffrent manga?
The waitress and her dad, as well as yoshioka are all from sorenari ni shinken nandesu. The cross dressing characters are new.