]Caroline certainly doesn't think that this country girl with the low connections and the family that raises impropriety to new heights has any chance to be considered for the position of mistress of Pemberley, so I don't see this as a defensive action.

At this point, her belief in eventually 'catching' Darcy seems to be high, but her behaviour towards Lizzy, and all the catty comments she makes about her show that she is jealous, and a little bit insecure. So, I think, she will utilize every opportunity she gets for reminding Darcy how unsuitable Lizzy is for him.