I think you're just taking the artist's tastes and trying to layer some non-logic on top of it. They just draw what they like, the above logic doesn't even make sense in an outlandish way since the things it could imply, even at a stretch, are in fact the opposite of what you list, e.g. her being very shy makes her more childish not more adult.
This artist drew her as a "bitch gal" (to quote him) because I guess he finds that more sexy, or because of the fact it's the exact opposite to her personality and everything she stands for as a character.
Teclo said: "I guess he finds that more sexy or because of the fact it's the exact opposite to her personality and everything she stands for as a character."
Its both usually. You do get the point, you just analyzed it wrong.
are in fact the opposite of what you list, e.g. her being very shy makes her more childish not more adult.
Which a consequence to her being socially awkward and emotionally repressed. Hence, why with similar characters, they're drawn a lot as closet deviants and/or crude and vain when they're comfortable. You can even see it officially like Elly in Milky Holmes, Yukie in Majikoi, or Meru in Sayanora Zetsubou Sensei.
Though to get things across straight, OOC Yayoi is a fad in itself from how her character is. Look at Joy Ride's other pics of her. Look at the recent pics of this day. Heck, look at Itsuki/Sunshine's odder fanart where she was just socially repressed and emotionally awkward instead.