不只一点/不止一点 mean not just a little 不只/不止 - not just 一点- a little 啊 is just ah~

means much, I am very happy.

actually, this sentence is a little awkward here. maybe the following sittuation is better. A: Are you happy? 你快乐吗？ B: only a little bit. 只有一点点。 what about you? 你呢？ A: not a little. 不止一点啊. i am delightful.我很高兴。

Thank you for explaining in details.

一点 means a littel. 不只 means more than or not 不止一点 “more than a little" "not a little" in this dialogue ,不止一点 means i‘m very happy.