HumanThatExists

It is legal. Legality is not decided by whether or not it is intended to be done, it's determined by the Pokemon having features that can or can't be in the game already. If you had a Ditto with Roar of Time, it would not be legal (If you use the Rage glitch in gen 4, it is possible, but still illegal. This is because it has moves it is unable to learn without a glitch. Catching a Mew with a glitch is different because it has all the normal moves and stats a mew could have normally). If you are genning, you can make a Mew with any moves, any stats, any caught location, and anything else. If you give it things that it can't normally have, it will be illegal. Legality and legitimacy are two different things. Legitimacy is just whether or not you caught it within the game or used something else to inject that Pokemon. That is why a ditto from gen 4 with a ton of moves it can't normally have is legit but not legal (only through the Rage glitch). This means that, technically, a glitch Mew from generation 1 is legal and legit. Another thing that proves it is that you can transfer a glitch Mew up to newer generations and put it through wonder trade or something. There are a few things that can make illegal Pokemon tradeable, such as taking the ribbon off of a shiny event Genesect from gen 6 and trading it in gen 7, but that isn't how the Mew is. A glitch Mew is not an event with a ribbon and fateful encounter; it's just a normal encounter that you just had to use a glitch to get to and you can trade it without having to change anything about it. Edit: I also want to add the fact that a gen 1 Pokemon transferred to gen 2 became shiny based on its stats, so a shiny mew from the glitch would still be legal since it does not have preset stats when you encounter it.