Was Yomiza's art that bad? I think I can say it's unimpressive (Which is an odd idea to have an artist I generally think is good to draw plainly on purpose), but I think it's more resentment towards stealing his sister.
It kind of reminds me of how Takoha talked about bullies who are insecure about great art attacking it. Only instead of great art it's a guy who nailed Deido's sister. I guess in the end it's because he wasn't moved by the art that he doesn't understand why his sister would be, someone he idolizes to the point of a complex.