missing no is simply a "404" error in the game.
What i mean by that, is that each pokemon has a hex value assigned to it. There are 151 pokemon, but there are 255 HEXS, meaning there are 154 other possible values, aka missing no's. The game doesn't allow you to access these hexs unless you use a game shark to hard code the game to always render that hex (and of course the talk to the man in viridian city thing, which is a glitch)
Missing no is not a version of mew. It is not a version of uxie, pikablu, or any other nonsense. It's just a hidden part of the game.