Possibly, just possibly, if you add up everyone in the U.S. who consider themselves to be creationists or believers in intelligent design (including the many theistic evolutionists who would not impose their beliefs on the science curriculum) you might get up to that 75%. So far as "creationism" as portrayed in that pseudo-museum, I'm sure it would be a small minority. Minds that could convince themselves that that stance is viable might very well also convince themselves that 75% of the populace is on their side.