I've gone off the belching and farting

Current pre-mades and game-generated Sims aren't going to have the traits. Not sure if future game-generated Sims will get them randomly, or Sims born in game to families you don't actively play (if you have MC Command Center). You can always go and cheat the traits you want in or out; I could see maybe giving Bob Pancakes the Bad Manners trait because it would suit him, maybe Poor Emotional Control for Nancy Landgraab because she seems Angry all the time in my game. And I noticed that Sims that age up don't get traits in all the categories automatically -- when the teen aged to YA in the livestream she only had the emotional control trait. It depends on the parenting style -- you can emphasize certain behaviors, and others would be just neutral (for manners, neither good nor bad; you'll get the occasional belch or fart but it won't be a regular habit).