The infamous Pokémon MissingNo is not supposed to exist, it's a programming glitch and was never intended to be in the game. That being said there are no mistakes, just happy accidents, something like that anyway. Is MissingNo acknowledged as existing as part of the lore? I think that would be interesting, personally.
MissingNo. is not a Pokémon, it's not something that was never intended to be in the game. It is random data found in memory that is misread by the game, which believes a Pokémon should be there.
Nintendo's customer service for the GameBoy has a specific section about MissingNo., which says the following:
MissingNO is a programming quirk, and not a real part of the game. When you get this, your game can perform strangely, and the graphics will often become scrambled. The MissingNO Pokémon is most often found after you perform the Fight Safari Zone Pokémon trick.
To fix the scrambled graphics, try releasing the MissingNo Pokémon. If the problem persists, the only solution is to re-start your game. This means erasing your current game and starting a brand new one.
Even Nintendo calls it just a "programming quirk", it is not an entity whose "canonicity" can be confirmed or denied.