Going by what little Japanese I know, I'm guessing the joke here is that Taiga thought she was talking to Ami (calling her "Bakachii") and it turned out to be Sumire. It is kind of amusing since I did think their character designs were somewhat similar at first (and even saw one image here where the two were confused and was mistagged...)

I'm rather shocked they didn't make a gag out of it when they first introduced Ami because they do look strikingly similar and only the a slight different in hair and eye color.