Does the Ice Sickle drop in a corrupted ice biome?
The Ice Sickle can drop from the Armored Viking, the Ice Elemental, the Icy Merman, and the Ice Tortoise. All of those enemies spawn in every variation of the underground snow biome.
In fact, the only difference between a "pure" ice biome and a "mixed" ice biome in terms of monster spawns is the presence of the Pigron in the latter.