Mark Twain did write A Connecticut Yankee in King Arthur's Court, which was very popular, and propagated the myth of heavy armor being hard to move in. Twain lived in the Victorian age, where a lot of these myths were put forward.
It, like most of what Twain wrote, was satire. In this case, of the idea of chivalry.
Don't blame the guy. He was a product of his time, and one of our greatest writers. He traveled to England and I'm sure he was told by some museum curator that the stuff was too heavy to stand up in.
Blaming him is like blaming the director of Airplane for the NSA'a poor record.