this is done for laughs, and it's an irony and joke the show makes again and again, but really it highlights that the nobles and political factions xie zheng interacts with don't know him intimately. they know him as the marquis of wu'an and they know he's a great general and bloodthirsty and they fear he's like his uncle, but they don't know about the thing that make him human in his day to day, as opposed to a heroic figure people talk about.
meanwhile the first thing changyu learns about him is that he longs for his mother (it's the first word he says to her, and it encourages her to save his life), and it's something they have in common.
so the joke is in the vast contrast (the irony) between the marquis of wu'an and his life in lin'an as changyu's husband, but also in how little all these people know him.
like it's a joke, but the scenes between changyu and xie zheng are never a joke. their wedding, when he resets her foot, when he buys back the hair ornament her mother gave her, and everything else they share is never framed as a joke. it's all genuine and imbued with this kind of first love tenderness as they both realize their feelings while trying not to overstep the spoken boundaries they've set--this is an arrange marriage, he's leaving once he's all healed up.
even when he's trying to teach her that poem and she totally bastardizes it, it isn't a joke. at first xie zheng is bewildered and frustrated but then he's amused and charmed. changyu wears her heart on her sleeve and she's so, *so* sweet that it's impossible not to be charmed by her.
also, changyu's pretty sensitive about being a pig butcher. how she tries to keep changning from telling people that's what she does, the way she like wiped her hands on her clothes before touching her wedding robes, how she tells xie zheng to keep away cause of the smell, and how proud she is that her mom used incense so they never smelled like the work her father did...like it's a joke but it's not one the show legitimates, you know?
