"绝不是傻瓜" can be translated into English as "definitely not a fool" or "by no means a fool".
Both expressions convey the meaning that someone is not foolish at all. Here are some examples in context:
He's definitely not a fool; he knows exactly what he's doing.
（他绝不是傻瓜，他清楚自己在做什么。）
She is by no means a fool and can handle complex situations well.
（她绝不是傻瓜，能很好地应对复杂情况。）