True she is a cyborg but she is also a Japanese and her cyborg body has Japanese features as I explained above so its safe to assume she would nonetheless look like a Japanese woman.
IMO its not really what killed the movie, the movie apparently was shit otherwise anyway so a casting change likely would not have helped. But its ridiculous to say that the Major looks Western when she has typical features of a Japanese woman and a Japanese name.
When asked about the casting of ScarJo he said he didn't care because its common for Hollywood to cast whites in Asian roles, not the the Major's cyborg body was actually white. She's probably Japanese since she resembles the other Japanese characters more than the Western ones.