In the bowl of a stand mixer fitted with whisk attachment, whip cream on medium high, or until stiff peaks form. In another bowl, whisk crème fraîche to loosen it and then fold in whipped cream in two steps: Use a whisk full of whipped cream at first to lighten the crème fraîche, then fold in the remaining whipped cream. Reserve 1/3 of the mixture, cover it, and place in the fridge until ready to serve the cake.
Line a metal loaf pan with plastic wrap, allowing for extra to hang over sides. Place a layer of graham crackers in bottom of pan and then spread a layer of whipped cream mixture over crackers; follow with a layer of jam, and then another layer of cream. Continue layering: crackers, cream, jam, cream, crackers. Fold overhanging plastic wrap over crackers and place pan in the freezer for at least 4 hours.
When ready to serve, invert pan to release cake; remove plastic wrap and frost cake with remaining cream mixture. Top with pistachios.