For the cake: Preheat the oven to 350 degrees F. Line a standard muffin pan with paper liners.
Whisk together the flour, baking powder and salt in a bowl and set aside. In a small bowl, stir together the sour cream, tequila and lime zest and juice and set aside. Combine the granulated sugar and butter in the bowl of a large stand mixer fitted with a paddle attachment. Mix on medium-high speed until smooth and fluffy, about 4 minutes. Add the eggs one at a time, combining well after each addition and scraping down the sides of the bowl as needed. Mix in the vanilla. Adjust the speed to medium low and add the flour mixture and sour cream mixture in alternating batches, beginning and ending with the flour. Mix until just combined.
Fill each liner about three-quarters full using a 1 1/2-ounce ice cream scoop. Bake, rotating about halfway through, until a cake tester or toothpick inserted in the middles comes out clean, 20 to 25 minutes. Cool in the pan for a few minutes, remove to a rack, brush with the orange liqueur and let cool completely.
For the frosting: Beat the cream cheese and butter in the bowl of a stand mixer fitted with a paddle attachment until smooth. Turn the speed down to low and spoon in the confectioners' sugar until completely combined. Add the lime juice and a pinch of salt and turn the speed back up to medium high until light and fluffy. Frost the cupcakes with an offset spatula or by piping with a pastry bag once completely cool.
For the garnish: Cut the lime in half lengthwise and then thinly slice into half-moons. Top each cupcake with a lime slice. Combine the sugar, lime zest and salt in a small bowl. Sprinkle each cupcake with the sugar-lime mixture.