Captain America said: "Your suit... it's vibranium?" Why he used "it's" instead of "is it"? Is it right? Is it just an informal kind of conversation or is it just grammatically incorrect?

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I don’t know about it’s grammatical correctness, but it’s accepted in English.

When you say
“You don’t like it?” instead of “Don’t you like it?”
or “It’s new?” instead of “Is it new?”
it makes it less of a question. It kind of shows that you already know it, but you want to confirm it.
“You’re sure about this?” (You know he’s pretty sure, but you want to confirm it again)

It can also be a surprised reaction.
So if you think your employee comes to you saying he’s done with his task, you may say: “You’re finished? Already?”