I think most of us thought Cilan never cared when Iris called him "a pain" regarding his antics, until we realized all those times Iris has said it she was basically talking to herself. It was directed to Cilan but not at Cilan, this episode's the first time Iris has directed that comment right at Cilan. Guess having someone else who thinks exactly the same with Cilan's oddness as Iris (Dawn) is what caused her to directly speak out her thoughts.
From this (and a few other times it's happened) I realized Burgundy's the only person who's insults and harsh comments are unable to get to Cilan.
I found Dawn's sarcastic remarks towards Cilan's weirdness funnier than Iris groaning about him being a pain at the back. And from this I loved that Iris yells it at him because that's the first time Iris had a different reaction. Cilan was adorable when he started sulking and also when he recovered.
To those people who do this: Enough with all those unnecessary and mean phrases towards Kotetsu whenever you mention him (by name or not).
We get it, you don't like him, there is no need to keep shoving it in the faces of people who don't mind or like him. And seriously, just get over it.