Himeko is popular for her appearance and she isn't the only one. Since the 2nd season took the time to give the extra characters distinctive appearances as opposed to being a faceless and generic mob like a lot of other background characters, many of HTT's classmates have become surprisingly popular. Ichigo (the stoic red-haired girl) is popular for similar reason as Himeko
Himeko's basically the anime equivalent of Figwit from the Lord of the Rings movies:
http://en.wikipedia.org/wiki/Figwit