On the one hand, Mad Libs made up mostly of ordinary words like run, cat, short, and so on are very, very boring. On the other hand, trying too hard to make a silly result by using too many unusual words makes the result sound forced. I find that the better results tend to come from using words that aren't outside most people's vocabularies, but don't get used much in small talk, either. Ideally, you end up with a bizarre double entendre.