Weirdly, my take on it is the absolute opposite. Wearing your own merch in an amateur band just screams sad and desperate to me. It's way too "Butlins". If you're a well established, professional and famous band then it doesn't really matter. Your cool level is already well established, at least amongst your fan base. Although chasing "cool" is the most uncool thing any band or individual can do. So what do I know?