Sentence examples for a dummy from inspiring English sources

The phrase "a dummy" is correct and usable in written English.
You can use it in a variety of contexts, including to describe a person who is not very intelligent or competent, or an object which is used as a stand-in for something else, for example: "He was a complete dummy when it came to fixing the car; he could barely even open the hood." Or, "The astronaut was wearing a dummy of the new space suit for the photoshoot."