The bully trope is hot because the mother is being intimate with someone that causes pain to her son, so there's elements of sadism and cuckolding that mix there. But I agree that a mother being intimate with anyone can be hot. I think the hottest trope is the slutty mom, where she is very liberal with her sexuality in front of her son, but also indulges in sex with people that would be inappropriate in the eyes of her son. Check out stories by JayPierce95 on Literotica, where the slutty mom trope is used often, and the mother is engaging in casual sex with randos at parties or the club and whatnot, and occasionally in the presence of her son. That is prob the best and hottest scenario.