(B: should say, "I would have thought so." The most likely meaning here is that person B thought that Mary was a good girl, but that something recent has made person B not sure if that is true. That interpretation might change depending on context.

“你觉得今晚会有很多学生来参加烤肉party吗?" “我本来以为不会有很多人来，天气预报说会下雨而且学生们明天有个重要的考试。” 如果改成“I would have thought so"就变成“我本来以为会有很多人来，天气预报说会下雨而且学生们明天有个重要的考试。”这样前后意思不通。 有点像“ I think so"和“ I don't think so"那样的。我英语不太好，理解错了不要笑哈～