No one is criticizing her for having a flat chest. People are criticizing her because she looks childish. Is it nice to see a girl who isn't blatant fanservice? Yes. Does that change the fact that she is an amalgam of adult/late teen and childish aspects that create some sort of uncanny valley of age? No.
"uncanny valley of age"? I've seen girls over 20 who look like they could pass for 12-year-olds. I've also seen 15-year-old girls who look like they're in their mid-20's. You really can't judge a girl's age by her looks alone.
You cannot. That does not absolve the situation of creepy.
So it's creepy solely because you say so.
Well I disagree. I say it's not creepy. What now?
Well, now I judge you for wanting to date a girl who looks eight.
1: If you think she looks eight the core concepts of human physiology and puberty elude you.
2: Men are not allowed to date girls who look young without being judged?