Roasting someone requires you to know them pretty decently. You can call an acquaintance out on a few things, or make fun of a work friend pretty easily, but to really get someone jimmies rustled or to embarrass them pretty hard, you need to know them well. Unfortunately for the current generation of kids, their parents grew up in the peak roast generation and they are savage beyond belief. Basically, if these kids start getting fresh and begin stepping out of line, they're going to get made fun of in ways they didn't think were possible and will absolutely remember as much the next time they even think about stepping out of line.
Roasting is an art form, and these paint brushes have been well-worn, much to the disdain of these children.
To really roast someone in a way that gets everyone in the room groan-laughing, you've got to hit them where it hurts, and that involves getting personal.
It could be going after something that they're doing in the room, some way back childhood fear, or just something you know about their personality or attitude that gets to them. So if you truly know the person you're roasting, you could probably come up with some better insults about them than anyone else.
Which is why no one could roast you better than your Mom. Usually though, she's too busy raising you and making sure you don't kill yourself to take the time to put you on blast. But some people are just born savage, and becoming a Mom doesn't change that.