That's just his character setting
it's because he wants kyungsoo to feel that the baby is his too, that's why he said "the baby is ugly and i think she got it from you" something like that.
Because technically, it's not his. Although the other reply is also correct, it's just Chowon's character really.
And Chowon said that Bin looks ugly because she took after Kyungsoo. I guess Ch just want Ky to feel included (not in a best way tho, lol). Chowon knows that he is definitely pretty, so that's his way of telling Ky that Bin is his child too XD
Maybe it's because of how he grew where everyone always notices how he looks and actually wants the child to feel normal (like he said at the end) or he just don't wanna define beauty towards the child's appearance? Don't mind him he's just tsundere ಥ‿ಥ
I don’t understand why Chowon keeps saying the baby isn’t his and calling the baby ugly???? Cmon smh