The plain meaning of Cushi refers to someone of the Cushite Nation, and indeed there's a midrash that suggests that Moshe first married a Cushite princess before marrying Tzippora. With that said, "Cushi" was sometimes interpreted to refer to someone who stood out in their actions. There are a few examples I'm aware of:
- King Shaul:
"The response to this admonishment is found in the verse, as it is written: “Shiggaion of David, which he sang to the Lord, concerning the words of Cush the Benjaminite” (Psalms 7:1). Is Cush his name? Saul is his name. Rather, this is a designation that indicates: Just as a Cushite, a native of the ancient kingdom of Cush in eastern Africa, is distinguished by his dark skin, so too, Saul was distinguished by his actions, as he was absolutely righteous and performed many good deeds." (Moed Katan 16b)
- Michal, daughter of Shaul and wife of David:
"Rather, we must say that it is this tanna who maintains that Shabbat is a time for phylacteries, as it was taught in a baraita: Michal, daughter of Kushi, King Saul, would don phylacteries, and the Sages did not protest against her behavior, as she was permitted to do so." (Eruvin 96a)
- Baruch the Scribe:
One Scripture saith, "And Ebedmelech, the Cushite, said" (Jer. 38:12). Was it Ebed? Was he not Baruch, son of Neriah? But just as this Cushite is different in his body from all other people, so was Baruch, son of Neriah, different in his deeds and good ways from the rest of the sons of men. Therefore was he called a Cushite." (Pirkei D'Rabbi Eliezer 53:6)
- King Tzidkiyahu:
"The Gemara continues: Similarly, you can explain the verse: “Now when Ebed-Melech the Cushite heard” (Jeremiah 38:7). Is his name Cushite? Zedekiah is his name. Rather, just as a Cushite is distinguished by his dark skin, so too, Zedekiah was distinguished by his righteous actions." (Moed Katan 16b)
The People of Yisrael:
"Similarly, you can explain the verse: “Are you not as much Mine as the children of the Cushites, O children of Israel?” (Amos 9:7). Is their name Cushite? Israel is their name. Rather, just as a Cushite stands out because of his dark skin, so too, the Jewish people are distinguished by their actions, and they are different from all the other nations." (Moed Katan 16b)
- And of course, the same gemara suggests this understanding about Tzippora herself:
"The Gemara notes: Similarly, you can explain the verse: “And Miriam and Aaron spoke against Moses due to the Cushite woman whom he had married, for he had taken a Cushite woman” (Numbers 12:1). But is her name Cushite? Zipporah is her name. Rather, just as a Cushite is distinguished by his dark skin, so too, Zipporah was distinguished by her actions."