Icon was stuck as a black man though right? He couldn’t change forms? That’s a different case. Making J’onn the black rep, while well intentioned falls short because he can change to other races if he needs to. He’s been white in the past and if racists are coming after him, there’s no real reason he wouldn’t just change to pass if he had to. Now you could have it so that J’onn can only shape change in his Martian form and that he can only have one human form at a time, that form being a black man. Then that could work although it would probably be a little convoluted. But if J’onn can only pass as human in the form of a black man, that could provide some legitimacy to him standing in for black people, just like how Clark isn’t technically “white” but for all intents and purposes is white passing.