has a few loose screws

To have a few loose screws is the phrase which is used for people who act in a crazy way as well as being eccentric and silly. This is a common idiom which refers to a person who acts so different in a very exaggerated and eccentric way, that they seem almost crazy in the sense that their mental capacity is quite different from others. While they may not literally have a mental illness, they sometimes act as if they do.