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?”; Literal: “Because it isn't hard at all, is it?”

Key Points

1だって introduces the reason for not caring

Sacchan uses だって to justify why she is not listening seriously.

2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.

3じゃん seeks agreement casually

じゃん adds “right?” or “isn’t it?” to her dismissal.