Was Justinian a great leader ?
Yes Justinian expanded his empire by defeating the Vandals in Africa and the Goths in Italy. He created a codified law system, improved and/or built infrastructures and made great achievements in the fields of art and architecture.
He achieved a lot, but he was a bit ordinary as a human being.
Yes, as far as history goes, that labeled him Justinian the Great. He gets this title because he 'got the job done', performed his office at A- status, and set and saved and Byzantine civilization and probably saved European civilization.