Creationism, if that is the extent of your religious views, is irrelevant to most things this is true. (Well, except maybe if you are a geneticist or something). 

But it does tend to be comorbid with other stuff. You run across a hardcore creationist you have to start wondering what other hardline evangelical stuff do they subscribe to. Are they in the camp who believe certain things need to be achieved to bring about the end of the world? Because those people, the ones who want to help along the apocalypse, could make some poor judgements about middle eastern diplomacy. And if they are that evangelical, do they want to curtail the rights of others? People of other religions, or the irreligious, or fuck, even other brands of Christianity. 

That's my first concern when I hear a candidate is creationist. If they are into that, what else are they into, and what affect might it have on their behavior? You know, it'd be like finding out your candidate believes he was abducted by aliens. Like, that belief when taken alone is pretty irrelevant as well, but it can be taken as a symptom of other peculiarities which might cause something of a problem. 

And being a Brain surgeon doesn't really cast that concern away. I mean yeh, absolutely it is better to pick a guy with bizarre beliefs who is successful over a person with bizarre beliefs who lives under a bridge, but just because a bizarre belief hasn't hindered a persons ability to perform complex surgery doesn't mean it won't effect their judgement in leading a government.