SmartNotch is not distributed through the Mac App Store.
Follow these steps to open it safely.
Double-click SmartNotch.dmg. A window opens showing the app and an Applications folder shortcut.
Drag SmartNotch.app onto the Applications shortcut in the same window.
Right-click (or Control-click) SmartNotch.app in your Applications folder and select Open from the menu. Do not double-click — that won't give you the option to proceed.
macOS will warn you that the app isn't from the App Store. Click Open to confirm. You only need to do this once.