Rarity is not amused. And she gives a speech about why she is true American. Who believes in the American Dream of succeeding and bettering yourself. And how just because she speaks in a "British" (Mid Atlantic accent she corrects them) accent or she does not care for hot dogs and prefers caviar does not mean she is not American.
I like the idea that Rarity is actually very patriotic and even moreso than Applejack, but Rarity shows patriotism not through American symbols but through American values like hard work.