Yeah, as shitty as he was, it's likely what would actually happen if you suddenly tell a random medieval minded farmer about manga.
And it is still likely toned down cause he didn't straight out kill her like some old timey folks would have done to children they consider "failures".
Not saying that I think what he did was right, but simply that it is a cruel but realistic depiction of people's mentality in a medieval type society.
A lot of them thought "fun" was just wasting time that could have been used for work.
Medieval workers
Worked less than industrial workers do. Even during peak season when you worked sun up to sun down, you would have taken multiple unregulated breaks including an afternoon nap! Outside of peak season most work only lasted half the day, and there were tons of rest days or holidays. The linked article estimated that most peasants only worked 150-180 days a year.
Serfs couldn't participate in commerce or switch careers or anything. The Lord would claim a charge of their harvests as tax, but they'd be otherwise unsupervised within the Lord's laws. There was no concept of judging work by time spent, only by how much they produced.
That said~ There's no guarantee the Author is treating this fictional world like a historical analogue to medieval Europe. Eight chapters have not provided us with many socio-economic details. Either Nia's Dad is an outlier, or it's considered normal to sell off your daughters for bride price and tell them they're whores. I suspect we're getting a stereotype of a stern Patriarchal primitivism, as an allegory for the Stern Confucianist Asian-Parents trope.