In the sense he doesn't give a flying f* about face, appearances, and pride of those he doesn't respect.

those kids deserve it

He is disrespecting someone in their own home. Only a shameless person could do that, even if the other party deserved it.

Even just last chapter, the MC mentioned something regarding the guards intentionally being provocateurs to stir trouble, because he's trusting (and kind of gullible) enough to not pick up on it himself.

But it can't really be helped. He's not a schemer, which is to his benefit and detriment alike—as well as the MC's. If he was a more scheming individual, there's a chance he would have never brought the MC into his family at all, or eloped in the first place. And as seen in Chapter 14, when he mentioned having to cut his seclusion training short to take care of his sick mother, he has more or less always been like this; it wouldn't surprise me if even that had been a scheme to keep him from getting too strong in the past.

The MC is crippled and unable to learn martial arts, and so she's a schemer to survive. Her father is a martial arts prodigy, but too trusting for his own good—enough so that he ended up dying in only a few years. The two complement each other, by having her make up for his faults. Her showing a social cleverness unlike her father, yet a similar warmth as his, is probably one of the main reasons why Grandpa is showing her favor, too—because she is capable, and is helping to protect and instruct his son, even if it might be odd to think of it that way.