Even on legitimate key reselling marketplaces (like G2A or Kinguin), keys for older Rovio games are rare. If you do find one, it may be:
The game is now free-to-play with ads on mobile. You don't need an activation key at all—just download it from the Google Play Store or Apple App Store . Bad Piggies Full Game Activation Key
Do not trust key generators. You will waste two hours and potentially infect your computer. Even on legitimate key reselling marketplaces (like G2A
With the rise of services like Netflix Games (which now includes some Rovio titles) and Apple Arcade, the concept of a one-time "activation key" is dying. Game developers want recurring subscriptions or direct IAPs. Bad Piggies Full Game Activation Key