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Charisma will affect how well the idea is communicated, but isn’t necessary just to give fairly basic insults. A low charisma character can easily come up with the idea of saying someone has a little dick and that his wife and mother sleep around. He’s just a lot more likely to stumble over his words and sound unsure about it. He’ll deliver it in the classic style of a 12 year old playing call of duty rather than with the effortless wit of an insult comic.
I think they mean a real person, the player, would need a minimum of “12 charisma” to even think of saying something like that to the DM.
And the meme says “me with 8 real life charisma” y’know, so less than 12.