"Especially coming from a left-wing, pacifist, American, born almost 179 years ago, who seemed to switch his beliefs according to how he felt."
You say that like its a bad thing. What is so bad about being an American, or being a pacificist, or even - horror of horrors - left wing ( although it would be more accurate to describe him as an anti-imperialist) ? I rate him all the more highly precisely because of his anti-war and anti-imperialist stance. And what is wrong with views changing over time or with experience? That is a good thing, not a bad one - only the most zealous or close minded of individuals never changes their opinion.
He even became increasingly revolutionary, and supported the idea of the violent removal of the Tsar of Russia - so not always a pacifist either.
As to him being from 179 years ago - nowt wrong with using historical figures as a reference, provided you bear in mind that they - and in consequence their views- will be a product of their times. We are all a product of our times, our upbringing, our education and our life experience. The fact that he gained all of those things a few hundred years ago does not automatically render everything he said or wrote irrelevant.
So, some of their views might become redundant as society progresses and evolves, but equally others may well remain perfectly apposite.And you know, as people experience life, their views may well change. For a while he was an ardent imperialist, but later came to abhor imperialism.
It does appear though that Clemens had a particular blind spot when it comes to indigenous peoples, such as the native americans, or the aboriginals in Australia. He did view them as being sub-human, certainly in comparison to the "civilised" white man. I think above all else though he valued civilised behaviour and thought, rather than having an issue about skin colour.
Certainly he was an abolitionist, and favoured the rights of the oppressed over those doing the oppressing.
None of these things though devalue his comments about patriotism.
I notice you do not offer your own definition of patriotism, which is telling.