I think the housemates have identified Nadia as a good person with a strong moral centre. However much they individually profess to like/dislike her or find her too controlling they, at some level, know that she follows what is right, rather than doing what is easy or what makes you popular. Each group in the house knows that if Nadia is on their team it lends credence and a sense of decency to the values of that team. This is why so many of the arguments in the house are centred on with whom Nadia is friends. The Mean Girls team at some level know their behaviour is shabby and, even though they don't really like Nadia, they want her allegiance to make them feel better about themselves.