The Sugarplums are a group led by Dimentio in a dimension called Nimbus. They first appeared in World War X as rivals to Thanos and the Black Order. There are two branches, one residing in America and another in Japan, each worshiping a different "god". The American branch worships "Mickey" and the Japanese branch worships "Hello Kitty", which in reality are just masks for Dimentio to carry out orders.
The Mysterious Five Project
Hello Kitty's Sea Invasion is a stage in The Mysterious Five Project. Although it's never been confirmed by the creator, the stage could possibly be focused on a Japanese Branch Sugarplum invasion.