Do I need to? Well, OK. I suppose a gay, black Dr Who is a severe skew.
I'd disagree, but then a skew needs a baseline to be a skew, and I don't know what yours is.
Thing is: the Doctor is actually an alien, and we don't have access to the census data that would allow us to say what proportion of the Time Lord population is either gay or black. And that means we've left the 'losing credibility' argument behind, because we're no longer complaining on the basis of unrealistic representation, which earlier seemed to be the principal defence of this thread against accusations of racism.
For what it's worth, that's not an accusation I'm making.
How much any of this matters to any individual is fundamentally to do with what that individual's comfortable with. I grew up in a few different countries, I live in London, and I travel a lot; I meet and talk to lots of different people. Some of them are nice and some of them are pricks and as far as I've seen there's no racial or cultural correlation with being a prick, so I don't associate any negative feelings with other people's racial or cultural backgrounds. Like Doctor Who, my current next-door neighbour is both gay and black, and he's a real person, but his presence bothers me as much as the current Doctor Who does - i.e. not at all.
But I also appreciate that some people will watch telly programmes, or ads, and it'll jump out at them that many of the people and characters they see on screen aren't white, or straight, or whatever, and that will trouble them. There will be reasons for that, I don't know what they are for any given person and I'm not assuming, because I shouldn't. But those reasons are probably worthy of examination.
Also: invoking the 'measure twice, cut once' principle that all us woodworkers ought to agree with, the notion posted by somebody else on this thread that white heterosexuals are currently underrepresented, and don't enjoy 'equality,' in the media, is wrong and almost certainly the result of confirmation bias.