Could you also say: "不就是什么时候都是..."?
To me, the sentence 不就是什么时候都是你说什么是什么 sounds a little bit unnatural. I think 不是什么时候都是你说什么是什么?would sound better.
不就是 indicates that the speaker thinks what follows 不就是 is unimportant, or can be easily dealt with, etc.
For example:
不就是100块吗?给你就是。
Isn’t it just 100 yuan? Here’s the money.
不就是打扫屋子吗?包在我身上。
Isn’t it just a simple task of cleaning the room? I’ll do it. No problem.
男朋友:有个事想跟你商量一下
女朋友:商量,咱们俩之间有商量这个词吗?不就是什么时候都是你说什么是什么
In this context, does the female speaker think what follows 不就是 is unimportant? I don’t think so. She simply wants to satirize the man. And I think 不就是 may be not very suitable here.
In addition, 不就是 is also equal to 不正是.
不正是 means that something is the very thing mentioned previously, but I don’t see this meaning suits well in this context.
Just my personal opinion. Please correct me if it’s wrong.