theres a difference between spending time with a childhood friend, and a fairy king who kidnapped your fiance, is obviously flirting with her and outright goes "hey maybe marry me lol". aqua has only been jealous towards people with clear interest like the prince, and even then he hasnt taken it out tiara in any way other then being a bit more spoiled
also tiaras last fiance literally broke off their engagement in a very traumatic way, shes insecure and also naturally a very shy person. she also knows that aishira is the fl, and aqua is a capture target