The Mew itself is illegal due to the fact that it is encountered with a glitch. Therefore, Pokémon obtained from a glitch would be illegal as well. The process of making a shiny via 8F itself is not illegal, provided it is performed on certain Pokémon.
To further clarify, the only Pokémon you can legally make shiny on Smogon are ones that could be shiny in the first place. Standard encounter Pokémon don't seem to generate the required DVs to be shiny upon transferring through PokéTransporter. Stationary encounters, gift/in game trade Pokémon, fishing, and safari zone Pokémon are able to possibly be shiny, so a legitimately obtained one of these could be 8F'd to be shiny. However, injecting one of the legal shiny Pokémon into your box via 8F would fall into the 'legal hack' category, so while you could theoretically battle with it, you still wouldnt want to trade it. Hope that helps.