@SageSkye Azuma probably heard their conversation (about Mie liking Komura). Azuma was aware that the letter wasn't from Mie and Komura is still having a misunderstanding if Mie likes Azuma. (she doesn't.)
Azuma probably saw Komura getting flustered so he decided to give a "response" to Mie fully knowing that Komura will read it for her since she is unable to read it in the first place. So Azuma decided to put "Komura-kun also likes Mie-san" which hinting that he was having a misunderstanding. Resulting Komura-kun being wary of popular people.
tl;dr : Azuma helped Komura to clear the misunderstanding.