African Americans are noticeably overrepresented in contemporary American media, while Asian Americans are underrepresented. In the UK it is even worse where at least half of the actors will be Indians, Pakistanis and so on when the majority of the country is white. This is, of course, on purpose.

What (if any) data are you using? Because at a glance, both halves of your initial claim appear to be false.

Roughly 14% of Americans are black, and the measures I've found put black representation between 8-16% depending on which measure is used.

And roughly 7% of Americans are asian, and I'm seeing figures from 5% to 18% depending on measure used.