I guess I see it as Joyboy, not Nika, personally.
In the mural, he's surrounded by a army, and we know there was a big war during the Void Century in the Second World. And Emet the Giant Robot looks to be right there with everyone, who was friends with Joyboy specifically. So, I feel like Joyboy fits better than anyone else.
It could represent all three ages, I suppose. With similar events to what appears on the Mural happening during all three ages. Cyclical history repeating itself and all. During "our" Third World, Franky could be the Robot, Loki could be the Ancient Giant, and so on.
But, chances are, Joyboy and Nika are separate characters. Nika is likely the original inspiration for the Sun God during the First World. Joyboy is the first pirate that rose up during the Void Century in the Second World. That's my bet, anyway. They might be "reincarnations" of each other. Or maybe they just have the same Inherited Will, if there's a difference. But, chances are, they were two different individuals.
So, my personal bet is that the actual artwork of the Mural is depicting the events of the First and Second Worlds, on either side of the Tree, with the Third World not being illustrated, because it hasn't happened yet.