If you would like a thicker, creamier sauce add 1 c. sour cream at the end of cooking time and stir to combine.
Frozen meatballs have already been cooked before they are frozen, so there's no reason to brown them before putting them in the slow cooker. If you make homemade meatballs it is a good idea to brown them first before placing them in the slow cooker or they tend to fall apart.