News flash: This is a presidential election year. The campaign motto of the current front-runner for the Republican ticket is “Make America great again,” a call to action that both implies that the U.S. of A. isn’t so great anymore and raises the question: What was great about us in the first place? Jeremy Scott’s latest collection provided a few answers. Denim. Electric guitars. Rhinestone cowboys. Most of all, though, these rockabilly-inflected clothes paid tribute to outsize American optimism, the kind that accounts for the Space Race, on the one hand, and the armada of wannabe stars who have made their way to places such as downtown New York City, Nashville, and the Sunset Strip over the years, armed only with their big dreams and the sense that nothing, here, is impossible.