Any real fan would realize that logically, the Second Age SHOULD be more diverse than the Third.

Second Age human geopolitics was dominated by the Numenorean Empire, a colonial empire that reached from Arnor in the north, to Harad in the south, and encompassed vast territories in between. Numemor had started out with a population made up of whoever had survived the destruction of Beleriand, the "gleanings" of many tribes, whatever ethnicity or color they may have been. Then, like the Roman Empire in real life, their nation took over many others, and if their subject peoples had any rights, the Numenorean Empire would have been made up of many peoples of many nations, languages, and colors, and logically there should gave been a lot of immigration and intermarriage within the Empire.

There were also more dwarves than at the end of the 3rd Age, they had more cities and more power, and more of the "Seven Tribes" were around. Since Tolkien never went into detail about their various groups, there's no reason to assume that all of them resembled caucasian humans. And since Hobbits were always described as having curly hair, brown eyes, and skin that ranged from fair to brown, there's no problem with diverse casting there!

Really, the only problem with diverse casting would be among the elves, who usually invariable described as pale and fair... but im willing to allow diversity there too, just because it makes the racists heads explode. Fuck off out of the fandom, assholes! You're not real fans, you know shit about Prof. Tolkien, or what he believed in or the messages he was trying to put across.