Yes, you can prepare the strawberry mixture up to a day or two in advance and keep it well-covered or in an airtight container in the fridge. You can also have the phyllo shells baked, cooled, and kept in an airtight container on the counter until it's time to fill them up and serve. I do not recommend adding the strawberries to the mini cups until ready to serve so that they don't risk losing their crisp texture and becoming soggy.