Arduino Mega - Serial to DB9 plug wiring

Yes, that looks suitable.

The pin connections you listed are for the connector on the back of the PC, which has male pins. The Pololu board has a female connector and appears suitable for connecting to such a PC serial port, either directly or with a 1-to-1 wired serial extension cable with a male at one end and female at the other. In that case, this adapter will have pins 2&3 swapped from your pinout in the OP, (Billy's Law notwithstanding).