I've had my share of days reading Bulbapedia articles about glitch Pokemon too, its very interesting!
Now about Decamarks, if you read the top of its Bulbapedia page, it would state:
"It was programmed into the game to prevent game crashes and is technically not a glitch Pokémon because its information was defined by the programmers. However, ??? can also occupy other values of the 65536 slots available as well as cause glitchy behavior in games, some specific versions may even freeze the game, such as hex DEFD." - Bulbapedia
As stated, it can take many other versions or forms of itself, which in some cases freeze the game.
The rest of the Bulbapedia article doesn't really say anything about its effects in other games, but while it says you are able to transfer or trade it into Generation IV/V games, I think the article itself was representing a general whole of glitch Pokemon (its versions), and not a specific, such as missingno. And migrating data like this into other games (or trading), might change the glitch Pokemon or take on a different form (different versions from each game) because each game might read it differently.
You can infer this from the trivia section of the article page:
"There may be similar glitch Pokémon in the Generation IV games, but no investigation into this matter has been reported." - Bulbapedia