This is a ramble done on my smart phone, apologies for the incoherency:
Okay, I know I'm going to get flack for pointing this out...but the character she's playing is a brain living in a mechanical husk. She could literally be played by any ethnicity or gender for that matter and it would make sense for the rules of this universe.
In the original animated movie, she's very detached and calculating. She doesn't really see her body as a sacred vessel that represents all that she is and believes in. She sees her body as a tool to be used for her job. She doesn't really have a concept of modesty or shame for her form because she doesn't really care what she looks like so long as she can do her job.
I feel like this movie is going to be very Westernized much in the same way that Dragon Ball Evolution was. I get the feeling like this is going to be a bad sci-fi script repurposed and using only names and locations of the source material.
Now is whitewashing a character wrong? I don't think so necessarily, not with this character or this setting where she could literally look like anyone if she wanted to. Would it be more authentic to the source material if she was Asian? Again, maybe, maybe not. This movie is being made by Hollywood for Americans with American sensibilities. Dragon Ball E deviated heavily from the source material because some suit in a board meeting convinced other suits that no one was going to want to watch a proven best-selling story the world over, and instead, they had a better story in mind. One that tested really positive in all of the right demographics they were trying to hit.
Scarlett Johannsson got the role because she's off of the success of the Marvel movies. She's easily identifiable to the American audience this movie is being made for and most likely Dreamworks was nervous that their return on investment would be minimal if they couldn't get a highly recognizable actress to play the starring role.
TL;DR: Casting choice is a product of the Hollywood system. Whitewashing doesn't necessarily apply to the character of Kusuragi as she's a brain in a machine and can look however she wants. This movie will most likely pull a DBE and be completely unfaithful to the source material anyways so who cares if an Asian actress doesn't get her name dragged down with this production? Most would call me pessimistic, I prefer to call myself realistic.