Can you make Mini Fruit Tarts in advance?

Oh yes, of course. Although if you do that, you might want to brush the bottom of the tarts with a beaten egg mixture before you bake them (since you will need to bake the empty tarts before you add the filling, most frozen mini tarts are not baked.. unless you buy pre-baked at the bakery?) so that will create a film between your filling and the tarts, that way they won't get soggy if you leave them in the fridge overnight.