U ever tried Peach? Fox may have a good Dsmash but it's pretty laggy afterwards. Oh and Peach Dsmash is very fast startup, insane damage and minmal lag afterwards. You can string one after the next within 1/8th of a second.
Fox Dsmash is really only useful when u attack opponent with Fsmash knowing they're going to roll around you and then u attack with Dsmash immediately after u Dsmash to hit them in their roll. Works wonders he he. In actual fact it is his Usmash into Uair that is the best combo he has. And Usmash is better than Dsmash.
Oh the Grab dthrow attack very close off the ledge is crazy yeah - like Kirby's bthrow or fthrow off the ledge. Inescapable with Kirby, but Fox's is too situational to be useful.
However, the grab when they recover doesn't work if they grab just on the ledge - which is why I don't see that many foxes grab from the ledge unless they knew the player was going to get up onto the stage from the ledge without attacking or rolling. So again, too situational. Better off trying to do a Usmash/Uair against them.