Nvm about updating my previous post, I'll just make a new one.
I honestly don't know much about the actual match-up, but I'll just say random things in the match-up to be aware of.
-Being edge-guarded really sucks, and Marth can do it really well. I don't have too much of a problem of recovering around him, but when he decides to prevent me from getting back on-stage, I'm in trouble. Generally, Marth's F-air and N-air and variations of it (falling, rising, double jumped, ran off the ledge, etc.) all stop a lot of Diddy's options, so you have to read/predict what Marth is going to try to punish. He has a good option for whatever you try to do, so pick the one you don't think he's covering.
For example, if your opponent Marth usually SH F-airs or N-airs, try to time a regular get up or roll back onto the stage. However, if you're at 100% or higher (or the Marth has good reaction time) they can do the SH-Fair and still D-tilt/D-smash/F-smash your roll, so watch out.
Recovering from the ledge is pretty difficult. With a banana, this process is much easier, as you can use it to (obviously) throw at Marth. You can try to drop down from the ledge, and popgun reversal banana pluck or wall-jump banana pluck a banana to throw him off.
Be very aware of your positioning (AKA spacing) and don't spot-dodge or roll much. You'll get side-Bd, or D-tilted, or whatever if you do.
OoS Glide tosses or regular OoS tosses are really good in this match-up to punish the slight lag Marth has. If a Marth hits your shield with F-air, you OoS GT back, and he tries to upB or something, you'll be out of range of that (although I'm not sure of the likelyhood of that happening) and you can punish it.
Speaking of his upB, it really sucks. It gets out of basically everything. It can get out of your U-smash (which is pretty good in this match-up otherwise), punish your dash attack, punish your F-smash with good DI, and maybe your D-tilt ( not sure, haven't tested). Luckily, if he whiffs it, it's very punishable; a golden chance to single banana lock.
If the Marth you're facing doesn't know he can SDI out of U-smash, use U-smash, It's really good. Generally, you want to be under Marth and a little bit behind him, so he can only use his laggy D-airs and B-airs, and you can punish those and keep him in the air as long as you can. U-throw also fills this purpose.
Generally, keep him at mid-range. Pressure with peanuts and bananas (ideally keep a banana in your hand at most times to get better OoS options, and keep a 2nd on the field pressuring him), try to bait any form of attack, and quickly punish.
More later, maybe.