Why do guys think the fake, plastic girls are "hot"?

I just got back from driving my little sister back home after her first bar mitzvah. When she got in the car, she started crying so I asked her what was wrong. She said this girl who looked like a barbie doll kept flirting and hanging around her crush. I didn't believe it because my sister is absolutely GORGEOUS. Naturally pretty, doesn't need makeup or fancy hair. Perfect skin. I said I wanted to see for myself so i went into "get her purse" and sure enough there she was all over the boy my sister liked. This little 13 year old was wearing a dress up in her , and caked on makeup with this ridiculously high poof that made her look like Snookie from Jersey shore. Ugh, it was disgusting! Why do boys like girls like this? I noticed that boys are like this in my school too? What advice should I give her?