Come to think of it, I just assume that what Tojiko's wearing is just a simple dress... Here, the collar is not part of the dress, but actually comes from the dress shirt she wears underneath. Neat.
I guess it's also a play on Tojiko's famous "やってやんよ!" phrase. The original here is "やってらんねーよ..." – effectively turning what's basically "I'll do it" to "I won't do it".
