Hey, Lee. I just got your private message regarding this thread. I just started to actively utilize this account here, so I'm not quite used to checking for notifications here yet, but I hope you don't think I was ignoring you or anything.
I've had the opportunity of sharing a few friendlies with Texas' best Dedede player, Fogo, so most of my recommendations are going to be specific to this particular match-up. First things first: although I would say that this match-up would lean towards Diddy in his favor, it's not by a landslide like some people would like to believe. Yes, massive size makes Dedede more prone to banana barrages from more obtuse angles, but prudent Dedede mains will do everything they can to neutralize your banana game with his ability to swiftly close ground distance. And unless you're playing on FD, chances are you're facing off against him on a relatively small stage, so you're going to have to nimbly work your way around his pressure so you can adequately set up your bananas (as you no doubt probably realized for yourself).
Also, it is very possible for Diddy to smash DI out of Dedede's chain-grab. Allow me to clarify - it does not move you completely out of Dedede's range because of the the sheer distance he can cover with his dash speed, but it will create enough of separation between the two of you after his D-throw for you to jump, roll, or spot-dodge. I can almost always reliably smash DI by smashing the directional stick and the C-stick away from Dedede.
Try not to get too caught up in filling dead space up with empty movement that don't contribute to either trapping, spacing or mind-gaming setups. There were more than a few times where you went into what Vyse has appropriately labeled auto-pilot mode and just started blindly glide-tossing bananas into a shield, only for you to get punished with a shield-grab as soon as you were in range. There's a difference between weaving an illusion of unpredictable movement into your pressure game, and then just tech-flashing for the sake of it.
Of the three matches posted, two of them could have very easily been victories in your name. Avoid being cute and flashy all the time so you don't end up paying for making simple yet costly mistakes - you know what they are. Try making better and more frequent use of your more underused moves such as your infinitely useful tilts if you're concerned with move decay.
Finally, I'm in no way trying to discredit the difficulty of your matches against Florida's best Dedede representative, but he's certainly not the best Dedede I've seen. Frankly, there are a dozen of things he could have done with the moves and baiting strategies that he didn't use to take advantage of your match-up inexperience and cut most of these matches by about two minutes shorter on average. You're a very competent Diddy main though, and I'm sure that if you'll just take the time to be more deliberate and precise in your decision-making, this and future match-ups will become more of a breeze for you.