At first the sauce will be very thin, but as some of the moisture cooks off it will thicken up. Also, once you turn off the crockpot and let the meatballs cool a bit, the sauce will thicken up even more. If it still isn't thick enough for you, you can mix equal parts cornstarch and cold water (about 1 tablespoon of each is a good starting point) to create a slurry and stir that in to the hot sauce. After letting that cook for a few minutes it should thicken the sauce nicely.