What good comes from poking fun at someone who's already struggling? the_elizabeth is right, it just kicks them when they're down. Like in League, if a friend keeps losing lanes, joking about it won't help them improve. Or think about a coworker missing sales targets, making jokes just adds pressure. It shows you don't care about their feelings, and can even push people away from trying again. Better to offer support or just stay quiet instead.
Nah, sometimes a little ribbing is how people snap out of a funk. If my buddy is tilting in a game, a lighthearted joke can break the tension and remind him it's not that serious. Treating someone with kid gloves all the time just makes them feel more fragile. It's about knowing your audience, not banning all humor.
My friend Mark had a player in his D&D group who kept rolling terrible... one night he just snapped when we chuckled at his crit fails. Mark took him outside for a minute, bought him a soda, and let him vent. They made a new rule about no teasing during bad rolls... game night has been smoother since. Now that guy actually enjoys playing again, and everyone is more careful with their jokes.