I don't think it's really the same for one main reason: Robert is described as a huge big guy and clearly very strong, Cercei is a lot weaker than him, she has no way to defend herself. Joffrey is already as tall if not taller than Tyrion and a lot more physicaly capable (Tyrion is not only short, he's disabled, and can't fight as well as Joffrey). Tyrion slapping Joffrey is okay because it's clear that Joffrey isn't fighting back because he's a coward, not because he can't. Cercei, on the other hand, physically can't. She's helpless. If Robert was slapping Joffrey, the comparision would work.

That's my 2 pennies.