Historically, Greece contained a large number of ethnicities, and a large number of distinct cultures to go with those ethnicities:
- Albanian (in the Northwest, mostly Muslim, but also Christian)
- Macedonian (Slavonic)
- Muslim (Pomak)
Of those ethnicities and cultures, the Muslims left most of Greece in 1923, the Jews were mostly extirpated through the Holocaust, and the Christians were mostly assimilated to the Greek culture.
I think you can still legitimately claim that Greece is multiethnic, and that Arvanites, Aromanians, Slavophones, Pomaks, Turks, Roma, and Sephardic Jews are distinct ethnicities that are still identifiable in Greece. (That’s not to mention the substantial number of immigrants to Greece since the 1990s.)
But multicultural has come to mean something more. Multicultural has come to mean that, even if there is a dominant culture in the country, the state will not pressure its population to assimilate to that culture, and will accept the coexistence of different cultures in the country as an asset.
For better or worse, the Greek state has never been multicultural in that way.