I know it's a recurring gag but it seems Yamada might mature a little quicker than Kase. That said, Yamada could have reassured Kase by saying there's no way she'd be interested in Flower Prince yet she tells Kase there's no way a celebrity would be interested in her . I understand why that does nothing to assuage Kase's worries. Word choice matters when you're trying to effectively communicate with your partner.
Since Kase knows Yamada doesn't know how attractive she is and is a bit naive, it's natural that she'll worry Kase will be taken advantage of and want to protect her, especially since they spend so little time together. Kase is also insecure. Sure she's athletic and has her looks, but beyond that she's probably wondering what she has to offer a capable beauty like Yamada. All in all, they both hold their partner in such high esteem yet their partner doesn't see their own good qualities. I hope Kase will come to terms with her self and realize that Yamada loves her for her, not because she's a star athlete and a beauty.
As for Flower Prince, art is subjective so I don't think he should really enforce what he likes onto others.... but I guess this might be further lesson for Yamada in that she'll learn how to speak against people and fight for what she wants.