Well yes, that should be easy to do.
Thing is the look of the biome is only part of the idea here. The other part is giving the player early game access to spooky wood for builds during the Halloween season.
There's also the fact that a reskinned evil biome wouldn't allow you to plant it at your base without invalidating housing. The suggestion here it for it to be NPC friendly once separated from the naturally generated crimstone or ebonstone. Also a texture pack wouldnt be able to spawn ravens, Hoppin' Jacks, or ghosts.
