I've tried to create this capacitor, which according to the dimensions (10*13um) should be 268fF. met4 is connected to GND and met3 is connected to 3v3. However in the extraction it is only showing 20fF between 3v3 and GND. Am I using the cap wrong?

No. As a device, the device itself has the capacitance between the plates, and is only specified in the netlist as a call to the device subcircuit with parameters for width and length. The capacitance you see in the output is the (total) parasitic capacitance between power and ground (of which the cap bottom plate is a part).

oh, I see! thanks for clearing that up 🙂