Diddy's short hop fair beats out any of mario's aerial approaches so use that heavily in the match up. Learning to fast fall it correctly and auto cancel it allows you to use it as a bait to get opponents to get hit by a down tilt or whiff a move. Short hop fair will also hit a standing Mario.
As for the down thrown up tilt combo, at zero percent you can di down after the throw and shield. If the opponent reads this, they can re grab but most decent mario's will up throw to down air to back air or fair at zero. Luckily, you can air dodge at a specific time to get out of the Down air during this combo. This can also be read or reacted to however.
Banana is great against Mario even if they are cape happy. If they tend to cape too much, command grab will punish well. Learning to glide toss will also help to bait out a cape, throw you banana up or down, and then you can punish cape with a grab or down tilt. Practice using the banana as bait. Pluck one, but dont go for the banana, instead go for your opponent. Mix this up heavily so the opponent is always guessing if you will approach or grab the banana.
Whenever you know your friend will approach with nair, remember that your fair will beat it out. Also makes sure to tech the nair if he lands it because a missed tech will lead to a jab reset.