For me is for pity and fun, at the same time. But I don't discard the situation of "Make Mokou realize of something more important"
Yes, there's definitely something else that incite such provocation. Either that 'tragedy of long life' thing or Kaguya is somewhat jealous that Mokou is no longer has her at full attention. It's strange that Kaguya is taunting about impurity as she was the one who reject Eirin's offer to return to the Moon for sake of living on Earth.