From Tian's blog, Hanzi Smatter ("Dedicated to the misuse of Chinese characters (Hanzi or Kanji) in Western culture."):
Avril Lavigne was recently in Hong Kong for a one-night show. She obviously had help from someone with a magic marker.
The phrase 日本最高 on Ms. Lavigne was correctly written, but for a WRONG country.
日本 = Japan
最 = most, extremely, exceedingly
高 = high, tall; lofty, elevatedIt is translated as "Japan is the best".