If WW's been portrayed a certain way, as a fictional character, that's essentially what she is. Superman is the same. He's written to be an alien visitor from beyond, and logic will tell you that such a being clearly wouldn't look anything like the way he's depicted in the comic books. It's based on a fantasy, though -- not hardcore science-fiction.
From a purely visual standpoint, Diana of Themyscira is a woman of considerable stature with dark hair, blue eyes, and a skin complexion that ranges from fair to olive. Do such types exist in North America (the US and Canada)? Do such types exist in Latin America? Do such types exist in Europe, the Middle East and parts of North Africa? If you answered "no" to any of those questions, then you're wrong. The reality is that people from all parts of the world (including American whites, like Jaimie Alexander and Jennifer Connelly) satisfy the physical requirement for the role of Wonder Woman.
You should also know that we Americans are not some homogenous group; regardless of race or appearance, we all descend from immigrants. So, in suggesting someone like Gina Torres, and then, conversely, saying that an "American girl" shouldn't have the part, you're ignoring the fact that she, too, is an American. Even worse, whether you realize it or not, by using such language, you're sending a harmful message to non-white Americans that they are somehow unequal to or less American than whatever your perception of a typical American may be.
Whatever. No need to pull out all of these examples. I'm well aware of how she looks and is usually depicted. 
