I knew a girl with thin eyelashes in junior high/high school. She got made fun of for it a lot, by both girls and boys. So, I'm guessing some girls want thick lashes because they learned the hard way that guys do indeed care, and unfortunately society cares in general.
No, this is not a case of anyone caring about that specifically. This is yet another case of teenagers picking on someone and finding any difference to focus on. Teenagers can be horrible.