He defeated the previous owner of the Darksaber, and that gave him legitimacy to rule as Mand'alor despite being an outsider.

Source: https://collider.com/the-mandalorian-season-3-armorer-maul/

The fact that Grogu is walking both paths -- the way of the Jedi and the Way of the Mand'alor -- seems to be setting him up for big things.

Source: https://www.cbr.com/new-jedi-order-rey-skywalker-padawans/

Though he remained a bounty hunter for several years in Legends, Boba also became Mand'alor, going on to lead the Mandalorians in their fight against the Yuuzhan Vong.

Source: https://screenrant.com/star-wars-characters-after-original-trilogy-legends-canon/