Put the mascarpone, lemon curd, sugar and lemon zest in a large bowl and beat with an electric mixer until smooth. In a separate bowl, beat the cream until it holds medium-stiff peaks. Mix a third of the cream into the mascarpone mixture to lighten it, then fold in the remaining cream.
Stir together the limoncello, lemon juice and 1/4 cup water in a shallow bowl. Working one at a time, quickly dip half of the ladyfingers in the liquid, turning to coat both sides, and arrange in an even layer in an 8-inch glass baking dish. Spread half of the cream mixture over the ladyfingers and smooth evenly. Repeat with the remaining ladyfingers and the rest of the cream. Sprinkle some lemon zest over the top, then cover with plastic wrap and refrigerate at least 6 hours and preferably overnight.