B: says that no one ever became ill just because they didn't ask for good health on a tanzaku. And then:
A: Well, no one ever became rich just by writing on a tanzaku either (did they?)
I can't work out if んじゃないの is a real question or a rhetorical question.
Because of that I can't understand the reply. Is こまかいことを言うな "Don't say trivial things" or "Don't ask trivial things"?
In fact I'm not sure about the whole of B's reply. Is いいんだ supposed to be consoling as in "It's all okay. Don't worry about trivial problems", or is it more like. "It's fine! Stop asking stupid questions".