Combine the pesto and mayonnaise in a medium-sized bowl. Whisk in the buttermilk, a bit at a time, until you reach your desired consistency. Refrigerate until ready to use.
Toss the cooled pasta with cannelloni beans, arugula, pine nuts, sun-dried tomatoes, mozzarella pearls, and Parmesan. Drizzle with the creamy pesto dressing and toss again. Refrigerate until ready to serve.
