It is a violation of the End User License Agreement (EULA). It’s not typically a legal offense for the user, but it is considered piracy/cheating.
Players don't want to cheat to ruin the game; they want to cheat to enhance the experience. The Hellopet Mod APK promises exactly that: a stress-free, limitless creative sandbox. Hellopet Mod Apk Unlimited Cookies
If you’re interested in Hellopet (a virtual pet game), I can offer: It is a violation of the End User License Agreement (EULA)