The lyrics of the tune contain a simple message about boys’ and girls’ bodies having inherent differences and encourage children to find beauty and acceptance in those obvious differences.

This was about body positivity and acceptance of yourself. It's the correct way to deconstruct gender. It's basically saying don't worry about the stereotype that you think a boy or girl are supposed to embody, just be yourself because it doesn't make you any less of a boy if you don't embody all of the male stereotypes.

I highly doubt the song writer or Mr Rogers had any concept of modern gender ideology. Back then it was more about right wing stereotypes from a religious lense that wanted to force people to act a certain way. It doesn't seem like an accident he says the word "fancy" in the context of society policing boys to be macho.

Now the gender ideologues are saying don't be happy, listen to the stereotypes, your body is flawed.