How to use the Forward Air:
Usually considered to be one of Mario's worst attacks, indeed it's far from exceptional, but it has some good qualities that make it situationally effective.
The obvious use for it is ledgeguarding. It's a meteor smash, and considering that meteor smashes are more desireable in Brawl than they were in Melee due to the change in physics, you should always keep an eye out for a time to use this. Get in a good hit, and you will do maximum damage to your opponent's recovery even if they meteor cancel.
On stage, it's more situational due to significant ending lag. HOWEVER, this meteor does have autocancel frames. If you do a rising F-air from a fullhop, the entire attack actually ends before you hit the ground, allowing you to N-air or U-air, (or D-air, but don't do that) right as you land. Fullhopped F-air also autocancels on the platforms on battlefield, and considering how most meteor smashes, including this one have good hitstun, it could be a viable combo starter on people waiting on a BF platform. Even a SHed F-air can sort of start combos thanks to the great hitstun of the meteor.
F-air is also good for starting a techchase game on midair opponents. By slamming your opponent into the ground, you limit your opponent's options. Chances are your opponent will not expect this at all and will miss the tech. This is your chance to predict what they do, and punish them. I have comboed into Smash attacks this way before, and it's awesome.
And lastly, there is also the chance your opponent will have no idea how to react to a Mario that uses the F-air intelligently on stage, and since they will usually expect you to B-air, maybe you can throw off their rhythm by throwing this out more than they would expect. Many characters like to spotdodge and retaliate. However the F-air comes out a fair bit slower than Mario's other aerials, and has respectable duration, so it's not impossible you can punish an opponent's spotdodging habits with this attack.
I was sandbagging online with Mario, and throwing out F-airs just for the hell of it, and this is my experience with it.
As for the D-tilt, the best use I have found for it is using it on a stage like Luigi's mansion and spamming it on the columns to get around diminishing returns. Well, besides starting "combos" at higher percents. It also extends farther than the Jab I guess, and it can shield poke.