Does a dominant trait mean it is the most common trait? why or why not?

No, because  the definition of population is just a group of people. for example, if the population is that of sweden, the recessive trait, blond hair, is much more common than brown or black hair. but overall, dominant characteristics become more common over time. however, the dominant allele disease are extremely  rare, a lot  more then the recessive allele diseases.hope my answer has you : )