Adding to what others have said, SSJ being a transformation of enhanced power makes it more iconic than a title. Don’t think actually being the pirate king fundamentally changes Monkey D. Luffy as much as his journey does and it’s Luffy who is the icon, not really being a pirate king.
You had kids, teenagers, even adults worldwide trying to imitate the SSJ transformation. Cringeworthy stuff but it happened. Not really the same with Luffy reaching his goal.