In most countries, Missingno is largely not protected by copyright because most jurisdictions do not offer protection for works that are wholly generated by a computer, as they do not have sufficient human authorship. This includes the name "Missingno" and the classic d-shaped sprite, which is entirely generated by code and unidentifiable as any copyrighted Pokémon. Thus, Wikimedia's image of Missingno is identified as being public domain. Of course, any reference to Missingno as a Pokémon would be infringement, since those aspects are trademarked, so traits like the cry that's shared with Nidoran♂, descriptions of acquisition methods or game mechanics, or its forms that borrow real sprites could be considered illegitimate use.
Of notable exception are the United Kingdom and Hong Kong, which offer limited protections for computer-generated imagery for 50 years after their creation. This means that the d-shaped sprite could have some protections until 2047, but it won't cover the general concept of a glitchy-looking entity named Missingno if an original sprite is used (such as the one in this game) because it isn't considered a character.
So does this mean I can put Missingno's sprite in my commercial product if I avoid referencing Pokémon concepts and don't live in the UK or Hong Kong?
Probably, but Nintendo is notoriously litigious, so it's not impossible that they might try to take action anyway if your project was receiving a lot of attention or profit. It's hard to say, but it's possible that a court somewhere could be sympathetic to the argument that the sprite is consistently generated by proprietary code (even if unintentionally) or that you're misleadingly trying to take advantage of Pokémon's reputation for your own gain. Regardless of the actual legality, Nintendo and The Pokémon Company are very familiar with legal battles and would have the resources to drag the case out for longer than most individuals could deal with.
