Meekness is not a characteristic usually associated with leadership, much less the leader of a nation of well over a million people.
The Hebrew word for meek is translated as humble, poor and afflicted in the ESV. The word has a depth of meaning: 1) humble in mind (gentle) or humble in circumstances (needy); 2) unpretentiousness, a quality of sincere and straightforward behavior, suggesting a lack of arrogance and pride; 3) lowly, pious, modest of mind, preferring to bear injuries rather than return them.
Moses had an affinity for those who were afflicted. As a young man, he came to the rescue of the Israelite beaten by the Egyptian (Exodus 2:11-12), to the aid of the Hebrew being beat by his companion (Exodus 2:13-15), and the female shepherds mistreated by the male shepherds (Exodus 2:16-17). His lack of pride or arrogance was displayed when he expressed his unworthiness and inability to lead the people of Israel out of Egypt. In the event described here in Numbers 12, Moses is not the one offended by Miriam and Aaron's accusations. It is the LORD who defends Moses. Moses would have borne the insult.
As we have seen from Exodus, Leviticus, Numbers and Hebrews, Moses is a unique figure in the history of Israel. He is the only person in the OT described as meek. So the description of him being more humble than anyone else on earth is appropriate.
The Hebrew word for meek is used only 19 times in the OT primarily to describe an unnamed group for whom God has a special affinity. Psalm 25:9 is a good example of this group. "He leads the humble in what is right, and teaches the humble his way." Psalm 10:16 expresses the LORD's concern for the afflicted, "O LORD, you hear the desire of the afflicted; you will strengthen their heart." The word is also used to describe the Messiah in Isaiah 11:1-5, "With righteousness he shall judge the poor, and decide with equity for the meek of the earth."
Needless to say, just as there was One wiser than Solomon, there is One more meek and humble than Moses. Jesus' description of Himself in Matthew 11:29 uses the same Hebrew word. "Take my yoke upon you, and learn from me, for I am gentle and lowly in heart, and you will find rest for your souls." Jesus' action in Matthew 21:5 fulfills the prophecy of Zechariah 9:9, "Say to the daughter of Zion, 'Behold, your king is coming to you, humble, and mounted on a donkey, on a colt, the foal of a beast of burden.'"
King Jesus, just as Moses was an uncharacteristic leader because of his humility, You are more so. King of kings riding on a donkey colt, who would expect such a thing! The humility exhibited as You rode into Jerusalem on a donkey is minor when compared to the humility You displayed on the cross as the LORD of all creation. Jesus forgive me of my pride. How can I claim to be a follower of You without exhibiting Your ever present meekness, gentleness, and humility? You are the One willing to bear the injuries and insults. You are the Humble One.