「头铁」是什么意思？名词？形容词？

头铁 is not so exactly defined. I would go with numbskull here.

Seems related to 不撞南墙不回头:

这样的人注定会被撞得头破血流的。(or in this case virally infected)

The sense of obstinate in the above phrase comes from:

南墙指影壁墙。我国的建筑物大门一般都是朝南开的，旧时代有地位、有势力的人家大门外都有影壁墙，所以出了门就要向左或右行，直着走肯定撞南墙! (Chinese spirits apparently cannot go round corners, which is also why you find zig-zag bridges in China!)

头铁是指这个人很性格很倔，比如形容不撞南墙不回头，撞了南墙也不回头的人。

疫情严重，不戴口罩，美国人真头铁。
This epidemic is serious, 'Mericans are just numbskulls for not wearing masks.

numbskull: dull-witted or stupid person, originally meant 'your (good) sense taken by some palsy'

In the face of a highly infectious virus, refusing to wear the simple protection of a face mask is, in my opinion, dull-witted and stupid.

Now, if America was still a British colony ....

"头铁" itself is a "subject + predicate" structure, like "头痛", "脚酸"。

"头铁" means one's head is made of iron, so it is tough enough to hit anything, meaning one is tough enough to deal with anything and is not afraid of dying. Here "头铁", as a whole, is predicative or works as an adjective.

“美国人真头铁” means "Americans are really tough".

"不撞南墙不回头" or similar phrases like "不见棺材不落泪", "不到黄河心不死" means one goes in one direction and is too stubborn to take anyone's advice.

"头铁" usually has two meanings:

1. describe someone doesn't know the seriousness of something and still to do it.
2. describe someone has one-track mind, doesn't know to do simplify problem, and get his own way.

the meaning of "美国人真头铁" is 1st point above. Look at your pandemic situation!

頭鐵

Head iron: refers to people who are stubborn, obstinate, headstrong, stiff-necked, bullheaded, pigheaded, mulish, dogged, pertinacious

Many people use it in games when someone not afraid of dying always rushes forward.

The head iron refers to the person who is very stubborn in character, such as describing a person who doesn't hit the south wall and doesn't look back; or even hits the south wall and still doesn't look back!

The usage here is to describe people who didn't wear masks until the worst situation happened.

首先澄清一下这话没有恶意，更不是句脏话，顶多是有点调侃。

这是世界是最头铁的男人，抗击打能力最强。