You can definitely substitute whipped heavy cream for Cool Whip, but it will change the texture. If you use heavy whipped cream in the peppermint filling, it may curdle when freezing. Cool Whip freezes well, so for best results, it's recommended to use it in the filling if you plan to freeze this pie. If you're storing the peppermint pie in the refrigerator, heavy whipping cream will work just fine.