A minority can be biased and a majority that has nothing to do with who is overrepresented.
You can be biased towards Americans, although they are a minority (worldwide). You can be biased against Asians although they are the majority (worldwide).
Representation does not matter if you have no information about a group, yes.
If you have false information, you have to counteract that through correct information.
But it does not really matter how often a group is represented. You just have to counteract that bias somehow.
You want to counteract racist biases through movies in Hollywood. I doubt that this will help except for those people that think movies are real life.
I dont think representation does anything.
It maybe does something if you are already biased than it can have a effect.
If you have a racist bias against a race, and you watch movies where this race is only positively portrayed that can maybe have an effect. But those movies are usually boring. If you watch an all black movie, you have to have a villain otherwise it is boring and nobody watches it.
But you should not be biased in the first place,
And representation has nothing to do with bias, in my opinion, because you can also be biased against the majority.
And my suggestion is: You practice loving-kindness that is how you discard all biases. That is similar to enlightenment in spirituality. It is hard but it is possible.
There is a study about racial biases. But biases in general work in the same way as racial biases:
Brief loving-kindness meditation reduces racial bias, mediated by positive other-regarding emotions - Motivation and Emotion
I dont think that movies have that effect, except if you love the biased race and see kindness in the biased race. But that is the same as the recommended meditation.