One key facet to this song is the point at which it becomes the ending song, and how that timing and the song itself play against the larger theme of the anime. The driving message of Toradora is not so much how the characters evolve, but how all their concepts of love evolve.
In the begining, Ryuji and Taiga have very similar ideas about the concept of love, centered around the high school crush. In the episode just before the "Orange" takeover, we find out that Kitimura had the same notion, but when Taiga rejected him, found something new.
Throughout 'Mercury is Retrograde at Christmas,' we see the other main characters also begin to take up this transformation, along with all the pain and confusion that comes with it. From the moment Ami's question about her feelings of guilt rattles her to the point of fouling up the game, Minori is forced to consider her growing feelings for Ryuji and their potential consequences, in was she was not able to back at the beach house. Ryuji has his own moment of burgeoning crisis watching Taiga and Kitimura, while Kitimura has gone from tight containment to bouncing off the walls.
Taiga hasn't looked at herself clearly yet, but Christmas is the arc where we will see her do so. And as for Ami, she is doing her best to force things to a crisis, trying to make the fruits turn orange around her. None of them are quite ready yet, all a bit sour. But they are starting to see that quality within each other, and from there reflect what it means when turned inward. That looking forces them tighter together, unable to give up on those facing the same challenges they are facing.
The strength of this is that it is something that most viewers can also connect with. A challenge we have seen in ourselves. It garners an emotional catharsis that drives viewers to not let it go, because their sour fruit reminds us of ourselves, and we cannot toss that aside or stop from eating it... whichever of those two translations you like best.