Mystique isn't American? I don't think any of the versions of the character I've come across have ever had any kind of accent ('90 animated series, Evolution, Wolverine and..., the movies)?
Mystique was married to a German baron when she gave birth to Nightcrawler, so I figured she was European. But on further examination, I find that her original nationality is unknown. So I suppose she could
be American, but since she could be from a lot of different places and is known to have spent a lot of the 20th century in Europe, I'm not comfortable assuming she's from America. Given her long experience at changing identities, I imagine she's very good with accents too, so her accent wouldn't necessarily reveal anything about her origins. (Particularly in a TV or film adaptation, which can change a character's background and origins as needed.)