The knights' armor is clunky, heavy, hard to get around in, and often useless—such as in the scene where Hank unseats Sir Sagramor with a lasso:
This time you should have seen him come!—it was a business trip, sure; by his gait there was blood in his eye. I was sitting my horse at ease, and swinging the great loop of my lasso in wide circles about my head; the moment he was under way, I started for him; when the space between us had narrowed to forty feet, I sent the snaky spirals of the rope a-cleaving through the air, then darted aside. (39.7)
The brains of the people who wear the armor are pretty much the same. They can't think on their feet, they don't change direction easily, and they tend to stagger out wallowing in the mud and the filth. Hank notes:
There did not seem to be brains enough in the entire nursery, so to speak, to bait a fish-hook with; but you didn't seem to mind that, after a little, because you soon saw that brains were not needed in a society like that. (3.1)
Hank—he of the clear eyes and sharp mind—doesn't use the armor if he can help it. He's so into this fact that he goes on about for an entire chapter: In fact, he goes on about it for an entire chapter:
First you wrap a layer or two of blanket around your body, for a sort of cushion and to keep off the cold iron; then you put on your sleeves and shirt of chain mail—these are made of small steel links woven together, and they form a fabric so flexible that if you toss your shirt onto the floor, it slumps into a pile like a peck of wet fish-net; it is very heavy and is nearly the uncomfortablest material in the world for a night shirt, yet plenty used it for that—tax collectors, and reformers, and one-horse kings with a defective title, and those sorts of people; then you put on your shoes—flat-boats roofed over with interleaving bands of steel—and screw your clumsy spurs into the heels. Next you buckle your greaves on your legs, and your cuisses on your thighs; then come your backplate and your breastplate, and you begin to feel crowded; then you hitch onto the breastplate the half-petticoat of broad overlapping bands of steel which hangs down in front but is scalloped out behind so you can sit down, and isn't any real improvement on an inverted coal scuttle, either for looks or for wear, or to wipe your hands on; next you belt on your sword; then you put your stove-pipe joints onto your arms, your iron gauntlets onto your hands, your iron rat-trap onto your head, with a rag of steel web hitched onto it to hang over the back of your neck—and there you are, snug as a candle in a candle-mould. This is no time to dance. Well, a man that is packed away like that is a nut that isn't worth the cracking, there is so little of the meat, when you get down to it, by comparison with the shell. (11.8)
To be clear, that's only one paragraph from the chapter. This is more than just physical for Hank the Yank: his mental nimbleness—something that is demonstrated through his critical relationship with armor—sets him apart from the plodding tin-can-laden knights around him.
As Hank says, "brains were not needed in a society like that, and indeed would have marred it, hindered it, spoiled its symmetry—perhaps rendered its existence impossible" (3.2). He contrasts that later when boasting of his own smarts: "Here I was, a giant among pigmies, a man among children, a master intelligence among intellectual moles: by all rational measurement the one and only actually great man in that whole British world" (8.6). He's not always humble, but you can't accuse him of lying… or just following the status quo.