>>280221926
Aside from being an Aztec Goddess, Lucoa actually is one of the Earth's creators in the series, so I'm guessing she has some investment in not seeing it destroyed. She probably gives Tohru and Fafnir a pass because she knows they'll mostly keep to themselves. And she did take steps to make sure Iruru was actually pacified in the chapter where she met Saikawa.
Now as for these other dragons that Kobayashi is letting in that she doesn't know the threat status of? We might start seeing Lucoa getting proactive in dealing with them.