Theory: Lubba was part of Rosalina's Family

Abstract
In the past, Lubba was a part of Rosalina’s family on the Comet Observatory.

Arguments

 * Argument 1: Lubba knows Rosalina from “forever” ago, according to his dialogue in Super Mario Galaxy 2.


 * Argument 2: Lubba points out that Rosalina hasn’t changed since they last met, perhaps implying he has changed since they last met, with him having been closer to the appearance of the Baby Lumas she frequently takes in.


 * Argument 3: When Mario feeds Lubba a Star Bit in Super Mario Galaxy 2, there’s a chance he’ll say it tastes like Star Cake, which is something Rosalina is known to have made for the Lumas in her family using Star Bits.