I think calling it corporate media is more useful than mainstream, because it allows the role of the media in capitalist society to be made explicit. From a Marxist perspective the distinction between the BBC and CNN isn't that important, because the BBC is the state media of a capitalist state and thus serves the ends of capital in the same way as a corporate media company. You could just use the term 'capitalist media', but I think that would be more confusing to people who weren't already pretty radicalised