Dumber than a rock on a bad day
Dumber than a rock on a bad day, which would be dumber on the dumb scale compared to "Dumber than a rock".
Example:
Friend 1: Your dumber than a rock!
Friend 2: Your dumber than a rock on a bad day!
Friend 1: Shit.
Friend 1: Your dumber than a rock!
Friend 2: Your dumber than a rock on a bad day!
Friend 1: Shit.