Beat first five ingredients on medium speed until blended, 2-3 minutes (do not overbeat).
Whisk lemon juice and sugar in a shallow bowl until sugar is dissolved. Quickly dip eight ladyfingers into lemon juice, allowing excess to drip off; place in a single layer in an 8-in. square dish. Spread a third of the mascarpone mixture over top. Repeat layers twice.
Sprinkle with lemon zest. Refrigerate, covered, at least 2 hours before serving.
