She's an unusual person primarily known as an actor. I don't know if she can sing, but I know there is more to her than meets the eye (though what meets the eye is not exactly unpleasant).
That's the problem when you have a high profile job, people assume everything else you do is just self-indulgent wanking. And usually it is. But I remember seeing model/actress Milla Jovovich sing (original songs) maybe 10 years ago, and I have to say, it was really fucking good. Not just good-for-a-model good. Then again, maybe I was just drunk enough to enjoy it and it really wasn't so great. All I have are my cloudy memories.
So I reserve judgment on miss Johansson's abilities, but the odds are not in her favor. Hard to be taken seriously, those types.