What do you mean the skull is a melon?

A little silly is a colloquial expression, which is usually used to describe a person's state of unclear thinking or improper doing. The word "skull" represents the brain, and "melon" means that the brain is not in a normal state, a bit like "melon seeds". Therefore, "the brain is a little melon" can be understood as "thinking a little confused and doing things improperly". Usually, this word is used for joking or relaxing occasions, not for formal occasions.