Okay so girls are "People who say they are a girl" and also .. do you mean that "girlishness" is "the things that are most commonly found among people who say they are girls"?
"A girl is anybody who says they're a girl" feels weird, like it doesn't map onto my understanding of girl. I'm imagining a the most masculine man possible who says they're a girl to enter a woman's-only area, would you consider them a girl? (i know this is extreme example)