Isn’t it right to say “she’s not good”?
"Not good" and "no good" are both grammatically correct, and can usually be used interchangeably. However, "no good" sometimes implies that something is worthless, useless, or in this case, dangerous. "Not good" may be less strong and implies that the word "good" does not apply to something, but it may not be completely worthless. You could use "she's not good" in this case if you wanted to, but "she's no good" is a little stronger.
Get more Native English Expressions >