I think I've only had one or two atrocious sims in my game and they were pretty bad, getting into fights, not just the occasional rudeness now and then. If you leave sims completely to their own devices and never tell them what to do, their reputations are likely to be mild or neutral. I, on other hand, tend to play Sims as a happily-ever-after simulator, so I push my sims to engage in friendly socials because, in many cases, that helps them achieve their goals. So I end up with a lot of sims with great or pristine reputations. I don't find that it's actually harder to make a sim atrocious versus pristine. I just tend to direct them to nice socials more often than mean ones. You might be doing this too without realizing it.