How to design a switching circuit with minimal voltage drop?
You’ll need a device with a low RDSon and a relatively low threshold voltage.  You’re currently dropping 230mV although we don’t know how much current the SG90 is drawing when that measurement is made.  BSS84 are P-channel FETs and claim an RDSon of 10 ohms, which is very high if you’re going to use one as a power switching device.  You might consider a BSS308 with an RDSon of about 0.06 ohm.  Either way you’re likely to struggle with an SG90 running at 3.3V as they require 4.8 to 6V.