After WWII, Manga during this time was used to better the morale of Japanese citizens, by becoming a source of cheap and educational entertainment. Which led to the popularization of kawaii characters. Japan used to better their image to western audiences. This created a foundation for the importance of kawaii culture in Japan as the adorable characters were there to better hide the rebrand Japan was going through during its post-WWII era.
The economic boom that Japan experienced in the 60s and 70s helped further push kawaii culture into the spotlight, as well as with the rise of the kawaii icon herself, Hello Kitty, opening the door to trade with the west again. Creating economic importance as they desperately needed a sort of stability after the widespread poverty and devastation in Japan.
The 80s and 90s Japan further established themselves as the Kawaii culture capital. As with the rise of Tamagotchi and the rise of Harajuku fashion both became a worldwide sensation. Opened doors for them in Japan and outside, such as icons like Katy Perry influencing western audiences.
Further Research: https://digital.sandiego.edu/honors_theses/80/