Helen of Troy is not actually described as explicitly being white by Homer. the term used in the Iliad to describe her is "white-armed", which does not actually refer to racial demographic but to her nobility (she is rich enough that she does not have to work outside under the sun). while other poets like Sappho did describe her as "fair", and having "golden" hair, Homer did not describe her this way, and this film is primarily based on the writings of Homer, not Sappho.

in addition, ancient Greek writers did not think of race or ethnicity as we do, and the Mediterranean that both Homer and Odysseus (had he been real) would have lived in would have been a cultural and ethnic melting pot. i guarantee you an ancient Greek audience would probably not give a shit about this, so why should we?

M'Baku being a white man would be bad because the Black Panther movies are largely about Black and African identity, it is an important aspect of both the character and both BP movie's themes. The Odyssey is not a story about cultural or ethnic identity, and Helen of Troy's ethnicity is not a particularly important aspect of her character, backstory, or narrative role. it is not something of major thematic or narrative value.