Black Panther is one character and Wakanda is one nation created for an already existing Marvel universe. Stan Lee and Kirby realized that their comics lacked African representation so they remedied that by creating Black Panther and his background. Tolkien's Middle-Earth is much more than that.
His first collection of short stories, The Book of Lost Tales, initially started out as a "mythology for England" but that was abandoned for grander mythology that wasn't limited to English culture by the time Lord of the Rings was written. We're talking Indian, Byzantine, Egypt, Nordic, and Slavic cultures. If anything, the biggest influence on his work was his Roman Catholicism (his words, not mine), which I'm sorry to say is not a product of English culture at all. Kind of ironic considering how Europe and America portray Jesus as a white man with long hippy hair and a goatee when in reality he was a Palestinian Jewish prophet that spent most of his time in modern-day Israel and Palestine. But I digress. Ultimately, Middle-Earth is more of an epic exercise in world-building than it is a Mythology for one nation.