I must say, I do not find Kaya to be a very interesting or compelling character, but I think part of this is intentional & meant to portray her as a very sheltered character without much life experience or depth of character. She more or less believes anything she is told by anyone she perceives as an authority figure, namely her butler Klahadore. Even though she does argue with him, this is mostly shown as her being naive & immature even though he is in fact an evil character. Despite him being somewhat antagonistic towards her goals, she still believes him to be a positive influence on her life, shown by her obtaining a rare & difficult to obtain gift to give to him as a thank you.
However, she is smart enough to recognize that Usopp is a liar. While she typically has a very kind outlook on him, when he tries to warn her of the danger, she is quick to doubt him and even become angry that he thinks she is dumb enough to believe more of his lies. She makes it clear that she has always known that his stories are false but that they made her happy to hear, but now that he is telling the truth, it makes her uncomfortable and she pushes him away. This is a very complicated & nuanced dynamic that often happens in reality and I appreciate how it is shown on the show.
Usopp is clearly meant to be an analogue for the allegory of the boy who cried wolf. As he has repeatedly made false warnings of pirates invading the village, the other villagers simply do not believe him the one time he is telling the truth. This is another look at how reputation affects our lives and the way we interact with others is directly influenced by how they think about us based on previous experiences & knowledge. Even Kaya doubts him & one of her house servants is so angry with him that he shoot Usopp in the back while he is running away, typically a cowardly or evil move but no one really questions it when it comes to shooting a known liar.
I think we are again getting a glimpse of how truly immature & emotionally undeveloped Usopp is when he seems confused about why people don’t believe him. Even after Captain Kuro made a clear statement about how & why the villagers won’t believe him, and then he experiences it firsthand, Usopp still wonders to himself about the reason, which is very clear to the audience. Of course, despite this immature and unreflected personality, he is an honorable soul full of integrity as he desires to singlehandedly fight & defeat the pirate invasion, regardless of how the villagers feel about him. Luffy & his crew are able to pretty easily recognize Usopp’s pure heart and vow to help him defend his village. I think they would probably still try to defend it anyway, but they have a positive attitude towards Usopp nonetheless.
Last note, I noticed that there is a heavenly body in the background sky during this episode that sort of resembles a crescent moon, but it almost more resembles a sun being eclipsed by a moon or other object. I am not sure if this is meant to be an exaggerated moon shape, as the scene takes place around sunset and the object is still very high in the sky, or if in the One Piece world, the dark of night is formed by a daily partial eclipse of their sun, but it is very pretty to look at and adds a fun fantasy vibe to the overall setting.