When you're placing the three plots for your first three villager move-ins, the only personalities you'll find on the islands will be Lazy, Peppy, and Normal.
Your 6th villager will be a Smug that you get while progressing through the story. You're forced to take whoever he is, and it's completely random. After that, you can build plots for more villagers. Once you build plots, villagers will start appearing on the mystery islands again, and they can be any personality type. Be aware that if you don't find someone quick, a random villager may claim the plot you build.
You can change out villagers you don't like when they decide to move out. Alternatively, I've heard you can use any villager in the campsite (amiibo or game-spawned random camper) to force-swap with a villager of your choice.