Players in Pokémon Go have a mixed relationship when it comes to Pokémon in costumes. On the one hand, they’re cute and different from the original forms. They’re a great Pokémon to use during specific events. On the other hand, Niantic has made a majority of them for the starting evolution, and players cannot evolve them into any of their later forms. For example, if you were to capture a Squirtle wearing sunglasses, it wouldn’t be able to evolve, and it’d always be that Squirtle wearing sunglasses. With some Pokémon capable of Mega Evolving and the evolution only lasting a few hours, does the same rule apply?
For those who capture a Gengar wearing a costume, you can rest easy. A Pokémon wearing a costume in its final will still be able to Mega Evolve. So if you happen to catch Gengar wearing the Mega Banette costume during the Halloween 2020 event, you can Mega Evolve it into Mega Gengar and freely use it.
Feel free to capture the final evolution of any event Pokémon, especially if you want to use it later. Not only will it continue to wear that same costume, but you can Mega Evolve it whenever you want to raid with your friends or walk around with a Mega Venasaur behind you. The Mega Pokémon does not carry the costume over, though. So you won’t see a Mega Gengar wearing the odd costume, but when it returns to its standard form, the unique costume will remain there.