GameCube Controller for Pokémon Snap on Wii? Not Supported.

No. Pokémon Snap on Wii (via Virtual Console / Wii Shop Channel) only supports Wii Remote (sideways) or Classic Controller / Classic Controller Pro.

Why GameCube Controller Doesn't Work

• Wii Virtual Console titles do not natively read GameCube controller ports (except Super Smash Bros. Brawl, Mario Kart Wii, and a few others with explicit support).
• Pokémon Snap VC has no GC controller mapping.
• No official adapter enables GC → VC input.

Workarounds

• Wii U: Pokémon Snap on Wii U eShop supports GameCube Controller via GameCube Controller Adapter (USB). Best way.
• Wii Homebrew (Nintendont): Run GC version of Pokémon Snap (doesn't exist — only N64). Nintendont plays GC games with GC controller.
• N64 Emulation (Wii64 / Not64 / WiiSX): Play original N64 Pokémon Snap on Wii via emulator — supports GC controller via Nintendont's HID driver. Complex setup.
• Classic Controller Pro ($15 used): Best native Wii experience. ZL/ZR = shoulder buttons, analog sticks = aim.

Recommendation: Buy a Classic Controller Pro. Or play on Wii U / Switch Online (N64 app) / PC (Project64 / Parallel) with any controller.