Now that is very unusual, my first thought was that someone put that laminated card onto the label so that whoever used it knew what the Game Genie codes were. The reason for this is that the cartridge was most likely the initial version, and the codes for the future revision (1) most likely don't work for it.
The sticker came with Game Genie, you were supposed to stick them on the OTHER side of the cart, so you could read popular cheats through a clear window in the Game Genie itself. I didn't understand that when I was a kid and lost all my stickers. I CLEARLY WAS NOT ALONE IN MISUNDERSTANDING THE FUNCTION OF THOSE STICKERS!
BUT checking my Game Gear Game Genie BOOK, I can confirm that Gentlepoke is right and that there are two versions of the game! I'm not sure what the differences might be, aside from using different codes.
I'm not sure what the version differences are but Nemoide is right, it's just a sticker that came with the Game Genie that somebody unfortunately stuck to the front of the cartridge.
I've got a bunch of Gameboy games with those stickers on the back.