"zero fucks given" means "I don't care" but is a) profane and b) carries extra connotations. "das ist mir wurst" isn't crude in the same way at all, and I don't think it carries the same implication of keeping cool.

Translating idioms is hard and they rarely translate exactly, but especially if you pick an idiom which is already a niche way of saying something that there are more common ways to express.