The air tasted like a nickle. What does it mean?

Well, i suspect that the words you wanted were clouds and nickel. A nickel is a five cent piece, an American and Canadian coin which is called a nickel because it is made of a nickel alloy.

If the sky was "stacked" with gray clouds, i would take that to mean that there were lots of clouds, which were gray, and which reached a considerable altitude. If you can see that the clouds are stacked, though, it can't be completely overcast. I've never sucked on a nickel, so i don't know what they taste like.

Not knowing the context of the quote, it sounds like she's saying the air was laced with a smog which when breathed in gives the person a metallic taste in her or his mouth.

It sounds like the author is describing quite an unpleasant experience.

A nickel, having been handled by a lot of people might well have a salty taste. Given that this is a beach [ocean??] scene, that could be what the writer/speaker meant.

This is exactly how I read that sentence.