Grid password like the one Android lock screen uses can protect compromised users with keyloggers installed by adding an extra security layer with somewhat unusual input. To crack such password a hypothetical Chuck would need to log not only keyboard input but mouse input as well (which is possible, but such password still protects from less advanced keylogging).
If you're a keylogger victim, you're already totally screwed. A randomized PIN pad on ProtonMail's login screen is not going to help you because a PIN can be brute forced in a matter of minutes. An additional PIN step to the login process would only make it more complicated without improving security.
If you're concerned about keyloggers, etc, then you shouldn't be letting anyone else use your hardware and you should consider using a more secure OS like Qubes or Tails.
In order to protect against mouselogging, you should aslo randomized the layout of the items in the grid then since the layout will change each time the login page is requested, the mouse log will be useless.