I found the fact that he did lash out actually fit his character
He was in a very good place after the huge success in the school festival, full of confidence, that's why that letter of rejection came as a huge surprise to him. He felt that he was back at square 1. And then, as if he weren't angry enough, along came Mashiro, who not only managed to finish her manga in time, but also got to life her life a little (learn how to cook and stuff). I'm not sure, but I think that a small part of Sorata didn't want Mashiro took cook because if she'd learn how to cook and do other things quickly (and maybe better) than him, it'd be too much for him to handle.
EDIT: ^ Yes, he is