Anyone who has ever played me knows that I fullhop Dairs all the time. Fullhop Dair -> FF Bair/Uair/Nair depending on the situation, or a cape in midair to bait them into whiffing an attack and punishing that. The way Dair hits keeps them sort of "locked" on the ground long enough for me to force them to react to my fastfalled attack; often with a sidestep or dash in the wrong direction. Can't really be shieldgrabbed as I'm going up since it's Dair, and I always land behind them incase they try to shieldgrab my landing. Not exactly foolproof, but it's a good tactic to have in your pocket
Fullhop Nair works great as well, but slightly better for spacing than offensive purposes. I prefer Dair, but I've seen a few Marios use this well.
<3 Fullhop Dair.