Idioms are expressions that have a figurative meaning that is different from the literal meaning of the words. For example, "kick the bucket" means "to die," even though kicking a bucket doesn't literally cause death.
"Sitting on a cornflake" doesn't have a widely understood figurative meaning. It's likely a phrase someone came up with to describe a specific situation, but it hasn't become a common idiom.
Perhaps you meant to ask about a different phrase?