There are currently 3998 Touhou Project images and doujins, of which 2 are Marisa x Youmu, while in Pokemon Let's Go, there is 1/4096 chance of a Pokemon being shiny, ergo, it would appear that this ship is most likely twice as common as a shiny Chansey. However, using methods, it is possible to increase the odds of finding a shiny Pokemon to ~1/273, so it may actually be the case that the ship is ~7 times less common than shiny Chansey. It all depends on how you hunt for it, I guess.
