@randomname Spaghetti Napolitan is a Japanese original that used an even sweeter Soy-infused Ketchup derivative (often a "sweet and spicy" combination with Worcestershire, actually,) to compensate for low tomato supply, where Ketchup is already quite sugary. They've moved to a mostly benign tomato-based Ketchup, but Banana Ketchup (a Philippine import) is served in some restaurants, which is also tremendously sweet.
Among the Japanese, there is a lot of experimentation with the dish in Nagoya, which has a few gimmick shops that deliberately serve sweetened pastas. One that has built up quite a reputation is "Kissa Mountain," and you can see some quite similar dessert spaghettis in their portfolio, ranging in toppings and presentation. The one in the manga looks particularly close to Kissa Mountain's Sweet Matcha Ogura spaghetti, which is served hot.