How is it relevant? It points to his decision making processes, among other things. He looks at science and religion and concludes that creationism is a real thing that actually happened. I think that's relevant. I want a leader to be able to look at the same set of information that I have and reach a similar conclusion. When someone looks at the same information that I have and they come to a conclusion that is wildly different than my own, then obviously one of us is seriously mistaken. On this issue in particular, I think that all available evidence is so overwhelmingly on one side that I find fault with the decision-making abilities of people who take the opposite conclusion.
There is a fairly large group of people in this country who will not vote for an atheist for whatever reason, maybe they just don't trust them. I'm in the opposite camp, when people hold religious views that I find to be frankly ridiculous, and even contrary to physical evidence, I have a hard time trusting their decision-making abilities. I doubt that they can be trusted to make a good decision when they are faced with evidence that goes against their religious beliefs.
Paul's belief in creationism I believe is also tied to his views on issues like same-sex marriage and abortion. If he is president when a bill comes across his desk to legislate things like that, I don't think he's going to represent my views. At best he's going to say to leave it to the states, and at worst he's going to make something illegal on a federal level which I do not agree with.