Dastardly is an Aspiration Trait. All aspirations give your sim an extra trait so that they can complete their aspiration easier. Mostly this aspiration trait will give your sim certain moods or desires when performing a task related to the aspiration Deviance. Sims who take Deviance aspiration get the Dastardly bonus Trait, which gives them a higher success rate with their mean interactions. Mean interactions are not always highly accepted by other sims, which will make them dislike you very fast.

With the Dastardly trait you can trick other sims into thinking your a public mastermind and they will long more and more to see your new tips and tricks.

This is the perfect trait for Goofball sims as they are already very playful 99.9% of the time. It is up to you if you will step it up a little with the Dastardly trait.