I believe Shinji’s cello holds a lot of understated importance regarding the core themes of Neon Genesis Evangelion. Initially, it establishes that Shinji does still have attachments to his life, that he has motivation to learn and push himself to be better on his own accord. This is one of the few things he does without sharing his progress with others, without having to worry how they will perceive him because of it.
From the perspective of the viewer, it was incredibly sudden and surprising to see Shinji pour his whole heart into playing it, when, with nearly every other action portrayed in the anime, he is cautious and hesitant in his actions.
The repeated close-ups of hands and their motions during the anime (balling into a fist, grabbing something, letting go of something) are symbolic of the human capacity to… do: to make and create and hurt and destroy. Here, he uses his hands to create something beautiful.
His inhibitions are lost as he uses his hands to create something beautiful, contrasting the many other times we see him using his hands to cause destruction, upon instruction and pressure from others to pilot the eva.
His playing the cello is not the unhealthy escapism of listening to his tape recorder, which he uses to block out his surroundings entirely as a means of safely falling into his net of depression, but rather an open performance of his talent and emotion that is audible in his playing.
Here, he is forced to elaborate his emotions in a constructive way; consequently and unexpectedly, Asuka congratulates this nearly epiphanic action. The music reaches her in a way that Shinji’s words are never able to.
