Theory: Bumpsy Plains' Location

Abstract
Bumpsy Plains is in the Koopa Kingdom.

Arguments

 * Argument 1: It is the closest area in Mario & Luigi: Bowser’s Inside Story to Bowser’s Castle that is not a part of the castle path, and is the entrance to the underground tunnel that leads to Bowser Castle Path.


 * Argument 2: The grass in Bumpsy Plains is very dry and a more pale, yellowish green than the very green grass across the river at Blubble Lake and in most of the Mushroom Kingdom. This grass colour is similar to that of the war-torn Bob-omb Battlefield.


 * Argument 3: Possibly one of the biggest pieces of evidence is that all of the signs in Bumpsy Plains have spikes and a Bowser Emblem. These signs are not seen anywhere in the non-Koopa areas in the game.


 * Argument 4: There are hexagonal patterns on the ground in Bumpsy Plains that match the hexagonal patterns found on the ground in Bowser Castle Path.