I agree with you partially. Isis was an important part of the Hellenized Egypt. Serapis is a Greco Egyptian deity. Cybele was an important deity in rural Greece.
I agree, however, that Mithra was never popular in the Greek world. I imagine Mithra was associated with the Greeks' enemy Persia; thus, his worship was never totally embraced in Greece.
Nevertheless, the Roman god Mithras [note the added letter “s”] was worshiped throughout the Roman Empire, including Hadrian’s Wall. So, how did Mithras come to the Roman world? Plutarch gives us a hint:
“ According to the historian Plutarch, who lived in the first century A.D., the Romans became acquainted with Mithras through pirates from Cilicia, a province of Asia Minor. These were the pirates who constituted such a threat to Rome until Pompey drove them from the seas.
In his biography of this skilful general, Plutarch writes of the pirates: 'They brought to Olympus in Lycia strange offerings and performed some secret mysteries, which still in the cult of Mithras, first made known by them [the pirates]'. In the middle of the second century A.D. the historian Appian adds that the pirates came to know of the mysteries from the troops who were left behind by the defeated army of Mithridates Eupator. It is well established that all kinds of Eastern races were represented in that army.”
Interestingly, there is no evidence of Mithras found in the ruins of Pompeii (AD 79).
https://www.cais-soas.com/CAIS/Religions/iranian/Mithraism/m_m/pt2.htm