I believe that I have heard it is the second, a general mistrust of sorts.
Edit - From Wiki -
The phrase "I'm from Missouri" means I'm skeptical of the matter and not easily convinced. This is related to the state's motto of "Show Me," whose origin is popularly ascribed to an 1899 speech by Congressman Willard Vandiver, who declared that "I come from a country that raises corn and cotton, cockleburs and Democrats, and frothy eloquence neither convinces nor satisfies me. I'm from Missouri, and you have got to show me." However, according to researchers, the phrase was in circulation earlier in the 1890's.According to another story, the phrase was originally a reference to Missouri laborers being brought to Colorado to quell a miner's strike and requiring frequent instruction.