"Marceline's mom was revealed to have been black" THAT, that's the problem with this question. Saying that her mom is REVEALED to be black is like saying "Marceline's mom was revealed to have owned a green shirt"; it's relevance to the narrative is roughly equal.
Her mom wasn't "Revealed" to be black, she just was. The fact that we consider this innocuous fact important enough to discuss is exactly the issue.
Let me tell you a little something about Cowboy Bebop. Cowboy Bebop is widely praised for it's presentation; it's one of the most beautifully animated shows to ever come out of anime.
Another thing people point abut Bebop is how textured and exquisitely detailed it's setting is. A common criticism of anime is that if character's didn't have different clothes and hair, it would be impossible to tell them apart because they all have the same face and body type.
Bebop is the perfect counter to this argument; you could render all the characters nude and bald and tell them apart just as easily. The show's staff didn't do this to make some kind of political point, they did it because having your story take place in a believable world with lots of different kinds of people is just GOOD FUCKING STORY TELLING. Having everyone look the same isn't bigoted, it's contrived.
In Bebop, nobody comments on anyone's race, many of the character's just happen to be black, or native, or Japanese, or white. They don't treat it like a big deal because the whole point of the Civil Rights Movement was that it isn't a big deal; at the core, we're all human, we're all the same.
Let me ask you this; if you hadn't grown up in a world where 80% of all protagonists are white men, why would you EXPECT them to be?
Whenever someone makes up a main character who's black or a woman or gay or something, we get all up in arms about it like it's going against some kind of unspoken rule, and we never stop to consider how silly it is that we're surprised by something so mundane. We always assume that the writer has done so deliberately, like special care had to be taken in order to resist some cosmic urge to populate every story with people of the same race, gender and sexual orientation.
But we never assume the opposite; sure, we might give a writer some shit over everyone they write being male, white and straight, but we usually chalk it up to laziness or simply assume that the writer is also all of these things.
It never seems to occur to us that when someone is trying to forge a cohesive, believable narrative, having different people be different is just a natural thing to do. That maybe the overabundance of just ONE of the many races on our planet is proof that our fictional landscape is in an unnatural state, one that left it's own devices would just gradually disappear.