It's bad if an acting role which is specific to an ethnicity, culture, or gender identity is filled by a straight white person? But it's good if the musical 1776 or Hamilton is performed by a diverse group of actors?
In the ideal world we shouldn't bar anyone from auditioning for a performance of any role in a standard* play or film. And we should allow a director to intentionally cast against type if she wants a nonstandard* play or film.
But our world isn't ideal, so how do we deal with that? We know the available roles aren't representative of the world: too many sexy blondes under 30; too few women over 80, etc. We also know roles vary greatly in the degree to which they're specific to a particularl identity. So I think the reality is we work towards the ideal, knowing we'll never get there. You tilt the playing field a bit, or like a guerilla, advance here and withdraw there, testing the limits of what's possible, which means what people will support with money and time.
* "standard" means a play or film whose casting would be described by a critic without attention to the identity characteristics of the actor. So a director who wants to try a Shakespearean play with all male actors (as it would have been in his time) is doing a nonstandard version.