Homemade whipped cream can deflate within a few hours of being whipped if it's not stabilized. If you plan to just serve the whipped cream on the side, or the entire pie is going to be eaten immediately, this isn't really a problem. But if you want to spread or pipe the cream onto the pie in advance and trust that it won't weep or deflate, you need to make a stabilized whipped cream. You can find my stabilized whipped cream recipe here.