You say that, but it's not important with だって

Grammar Explanation: That said with だって

Yui hurries to her clubhouse to tell her friends, Kotoha and Sacchan, about a big case: there’s a cat that looks like a panda. However, Sacchan doesn’t engage in the topic and Kotoha continues playing her video game.

結衣(ゆい):: 「もおマジメに聞(き)いてよ　琴葉(ことは)もっ」; “Geez, listen seriously. You too, Kotoha.”
さっちゃん:: 「だって全然(ぜんぜん)大変(たいへん)じゃないじゃん」; “You say that, but it's not a big deal at all, right?”

Key Points

だって introduces the reason for not caring
- Sacchan uses だって to justify why she is not listening seriously.
全然〜じゃない makes a full negative
- 全然(ぜんぜん)大変(たいへん)じゃない means “not a big deal at all”.
- 全然 expects a negative ending here.
じゃん seeks agreement casually
- じゃん adds “right?” or “isn’t it?” to her dismissal.