The Dillinghams don't become content in their "poverty," but they show the wisdom to find ways of expressing their deep love in spite of their financial circumstances. They are called "The Magi" because they showed themselves to be "wise ones" in the demonstration of their love for each other, which is the most important thing in anyone's life.
Their low level of income meant that Della and Jim could not easily obtain money to purchase a Christmas present for the other that adequately portrayed the depth of the affection and appreciation felt. Della wanted to buy
Something fine and rare and sterling-something just a little bit near to being worthy of the honour of being owned by Jim.
To solve her dilemna, she sells her greatest treasure, her hair. The sacrifice gives her the wherewithall to purchase the watch fob that was "worthy of The Watch." Jim sells the family heirloom watch to purchase the set of combs for Della's long hair.
The sacrifices they made for each other gave testimony to their wisdom in putting each other first.
Of all who give and receive gifts, such as they are wisest. Everywhere they are wisest. They are the magi.