Commercial dumb is the idiocy the people in TV commercials tend to show. They say and do things that no one in the right mind would ever do in reality. And the others in the ad are so dumb that they don't even find that behavior bizare.
Typical commercial: Retard #1 Duhhhhhh!!! This cell phone from Verizan Wireless can makes me cool!!! I'm gonna pull down my pants!
Retard #2 Wow what a great phone!! Now we can text each other! (Not noticing that retard #1's pants are down.
A real example would be an AT&T commercial in which someone looking out the window thought he was watching HDTV.
TV viewer: Those guys are so commercial dumb.