The requirment for the minimum 0.1 ohm ESR is for compensation response. C1 and its corresponding ESR is keyed as part of the compensation network. Too quick or too slow of a response from C1 will cause compenstaion issues across the four process corners (Voltage/Temperature: High/High, High/Low, Low/High, and Low/Low). C1 can be a Electorlytic capacitor, with its corresponding higher ESR, providing it is stable across voltage and temperature. The base schematic for the SREG circuit in the PowerPSoC datasheet shows the ESR resistor can be external to the capacitor.