Accents aren't stupid!  Besides, maybe Mousse and Cologne just studied 
Japanese before they knew they would ever go to Japan, while Shampoo studied 
some other language instead.  Shampoo hatte gelernet Deutsch vielleicht. 
Happens all the time in real life. 

Cologne gave the explanation of not having an accent as something like "you
learn a lot in 300 years".

According to the Who's Who file, Mousse _does_ have an accent, thoigh it's
not a Chinese one.