No, the ACMarket app does not affect your device warranty. This is because ACMarket does not require any special modifications to work. It will run on your device in the same way that any other Android app runs and doesn’t flout any security rules laid down by the Android OS. However, some of the unofficial apps in ACMarket may invalidate your warranty so, if you ever need to take your device to a service center for repair, you should delete the downloaded tweaks first.