Like most static encounters in Pokémon games, there’s always a chance that the Pokémon in question could be shiny locked, which means you won’t be able to obtain its shiny variant right away.
Because Kubfu is a static Pokémon given to you by Master Mustard after completing his dojo’s trials, resetting for a shiny might not work.
As of now, we don’t know if the cutscene of Mustard gifting Kubfu to you actually shiny locks the new Legendary Pokémon, but this would line up with previous static encounters in Sword and Shield.
All three of the starter Pokémon—Grookey, Scorbunny, and Soble—were shiny locked at the start of Sword and Shield, so resetting the game won’t change the encounter rates at all. The same can be said of all NPC trades, Wild Area Pokémon that can’t be captured, and the Legendary trio of Zacian, Zamazenta, Eternatus.
Because Kubfu is a Legendary Pokémon and likely can’t breed an Egg, there won’t be a way for players to encounter a shiny Kubfu or Urshifu any time soon. Game Freak will likely wait a few months before hosting a Max Raid Battle event that will feature the Wushu Pokémon with a chance to encounter a shiny variant.
This article is being updated as more information becomes available.