No, rounding the average atomic mass in the periodic table does not always give you the mass of the most common isotope. The average atomic mass listed on the periodic table is a weighted average of the masses of all the naturally occurring isotopes of an element, taking into account their relative abundances.
Isotopes are variants of an element that have the same number of protons but different numbers of neutrons in their nuclei. Different isotopes of an element have different masses, and their abundance in nature can vary. The average atomic mass is calculated by multiplying the mass of each isotope by its abundance and summing up these values.
In some cases, the most common isotope may have a mass very close to the average atomic mass, so rounding the average atomic mass might give you the mass of the most common isotope. However, in other cases, the most common isotope may have a mass significantly different from the average atomic mass. Therefore, rounding the average atomic mass will not always yield the mass of the most common isotope.