The Shiny Rank is a meter of the Players efforts in-game. Each Rhythm Game has three large stars that are filled over the course of the song, depending on how well the Player performs. At the end of each round, the glitter filling the stars are exchanged for points, which build up the Shiny Rank.
The Shiny Rank feature is possibly based on the emblems the Player could obtain in the 3DS games to unlock new locations, features, and items from various shops.
By filling the bar, the Player will gain a new rank/level, and as they gain more, they will obtain special rewards, such as Chara elements or be able to use the ranking to unlock new stages.