I'm following TI's reference design (PMP11612) for a 5V 2A AC-DC adapter and was wondering if I would need to add overvoltage protection like a zener diode or a TVS diode on the input to this circuit? This design is to used in the US so the mains voltage will be 110VAC.
No, if by input (your unclear on this) you mean the AC side then the answer is no. The filter after the rectifier will stop most fast transients. Very large transients will blow anything out. Its just a cheap charging circuit.
A better place to put a Zener to protect the USB would be on the output to prevent an over voltage of more than 5V