1. In a deep heavy-bottomed pot, fitted with a deep-frying thermometer, heat about 3-inches of oil to 365º F.
2. In a large mixing bowl, using a hand-held mixer, combine mashed potatoes, ricotta, and sugar, season to taste with salt and pepper, until smooth. Gradually, fold in the flour, until just combined.
3. Working in batches, drop heaping teaspoons of the batter into the hot oil. Fry until golden brown, turning as needed to brown evenly, about 1 to 2 minutes. Remove fritters from the hot oil using a wire strainer. Drain on paper towels and season with a pinch of salt.
4. Serve with Herbed Sour Cream Dip.
For Herbed Sour Cream Dip:
In a small bowl, mix all ingredients for the herb dip together until smooth. Refrigerate, covered with plastic wrap, until ready to serve.