Yeah they can be good, but depends on what the pc is being used for. If it's not running into thermal limits it's much better off just trying to have decent airflow that won't suck up too much dust.

Laptops are usually designed pretty well when it comes to cooling vs. power when placed on a hard surface.

Your intake on the side would still be close to the carpet. Also outward and down would build up heat still

All laptops I have used suck air in from the base and blow it out the side/back. I like your upside down solution.