Theory: Yellow Streamer District's Locations

Abstract
Paper Mario: the Origami King's Sweetpaper Valley, Breezy Tunnel, and Scorching Sandpaper Desert (Locations of the Yellow Streamer District) are located in the Dry Dry Desert region.

Arguments

 * Argument 1: While the English name Sweetpaper Valley is quite similar to the Sweet Sweet Kingdom, this connection is not shared in the Japanese version, with the former area being called Wind Passage Valley.


 * Argument 2: For the Yellow Streamer District locations to be located in the Sweet Sweet Kingdom, a very large amount of off-screen travel would be required, making the Eddy River much longer than what the player sees in Paper Mario: The Origami King.


 * Argument 3: Shroom City has similar architecture to that found in Shy Guy Bazaar, a location already theorized to be in the Dry Dry Desert region, based on rugs featuring patterns depicting the Angry Sun and it having Green Cobrats. Both Shroom City and Shy Guy Bazaar also appear to be Mushroom People settlements that were later occupied by Shy Guy species (Shy Guys in the case of the Bazaar and Snifits in the case of the City).


 * Argument 4: The Scorching Sandpaper Desert appears to have its own smaller sun, likely similar to the Angry Sun found in the Dry Dry Desert region. This is indicated by the fact that it can fit inside a disco ball and a connection is implied when Olivia says “Wait, where’s the sun? Do you think it’s hiding away because it’s angry about something?”.


 * Argument 5: The Kingdom of Shrooms, which built the Temple of Shrooms and Shroom City, as well as the other ruins around Scorching Sandpaper Desert, also likely built Drybake Stadium and other structures found in Drybake Desert (AKA Dry Dry Desert in Japan). This is indicated by murals in both Scorching Sandpaper Desert and Drybake Stadium depicting the Toads that made them as much taller than they are in reality and using a similar art style.


 * Argument 6: The steps that were required to reveal the entrance to the Temple of Shrooms involved activating four Toad-shaped Towers, which each shot beams of light at the Temple’s location, revealing the way to find the entrance once all four were activated. In Gritzy Desert from Mario & Luigi: Partners in Time (another desert which likely connects to the Dry Dry Desert region), the steps that were required to reveal the entrance to the Koopaseum were extremely similar, involving the activation of four Dry Bones-shaped Towers, which each shot beams of light at the Temple’s location, revealing the way to find the entrance once all four were activated.


 * Argument 7: In Paper Mario: the Origami King, it’s stated that King Shroomses made his riches from the discovery of oil in Scorching Sandpaper Desert. In Paper Mario: The Thousand-Year Door, Lumpy struck rich after “discovering” oil in Dry Dry Desert.


 * Argument 8: King Shroomses was stated to be a frequent visitor of Shangri-Spa (located above the Great Sea, which is likely east of the Mushroom Kingdom). This would make more sense if the Kingdom of Shrooms was in the eastern desert.