People who joke about others' moms are most likely the ones dont respect their own mother, treating as they want to be trated. I dont joke about such a thing, and especially, i dont joke about such a thing with an (IG but still) enemy of mine... we're not close, we havent talk ever before, and you come here and joke about my mom = i take it as an insult its clear.
about growing up, idk how old are u and what you do in RL but prolly you dont have any aspect to tell me to grow up. This is completely about cultural elements, and seeing people say "grow up are u 13 or sth" clearly shows it. you think all grown up people should take words about their mothers from their foe as jokes, we think you still shouldnt but people till 10-12 year old can say such stuff as they dont exactly understand meaning of it, but after than point noone says such a thing, if they do that may be beucase of a serious hatred. We dont joke anywhere close to it anyway, but you're still trying to bring this to an age-maturity stuff which has no relevance to it. Tell that 'joke' to a random grown up man in Turkiye, you'll see what I mean.