Do all isotopes have whole number atomic masses?

The short answer is yes. The long answer is that isotopes have different numbers of neutrons than other isotopes of the same element, so when expressing the mass of an isotope, you don't take an average; you just add the number of neutrons and the number of protons. However, that number is not entirely accurate since the mass of a proton and a neutron are very slightly different. So, if you want to be very exact, then no, the mass of an isotope is not a whole number, but it is very, very close.