The Approach:
The number one reason Dedede is believed to lose this matchup is the amount of damage you take and level of difficulty it is to approach olimar. Me and fiction have derived a solid way to approach olimar. First and most importantly, you need to establish the mindset that the pikmin are not real(except purple). Ignore all pikmin that are thrown at you. Don't shield them, don't try to get them off of you, not even the whites. Run right through them and go to olimar. Take note of how many purples he has and where they are in the lineup! This is absolutely essential. You need to powershield the purples like you would falco's laser.
Now that you've gotten to olimar, you can notice that he has 2 or less pikmin because they are all on your body. Stop JUST outside of olimar's grab range. Most olimars will either dash grab or pivot grab at this point. When you get into the range I mentioned, throw out an ftilt. This will stop a dash grab or a standing grab and even an fsmash. If olimar pivot grabs take a few steps forward and start spamming that ftilt. At this point olimar is in a very difficult situation. He can either roll around you or try to jump over you. The best thing to do is expect the roll because it's harder to react to than the jump. If he rolls turn around and grab him and then backthrow him, you are now in a gimping opportunity. If he jumps, he's either going to try to jump all the way over you or he's going to attack your shield. If he attacks your shield with anything but nair, ftilt out of shield and repeat the process I just described. If he jumps clear over you follow him and wait for him to land. Try to grab or begin spacing your ftilt again. Olimar has absolutely NO safe options when you're close to him, and this makes the match very fun.
Dedede's advantage over Olimar's grab:
Olimar's grab is flawed and I completely credit fiction for figuring it out. As of now, the only dedede's that know this are me fiction and teba, and now you. TURN YOUR VOLUME UP WHEN YOU FIGHT OLIMAR!. You know that little sound olimar makes when he grabs? You need instant tech skill reaction time to it, start practicing your reaction to that sound. You either need to spot dodge on reaction or roll around olimar on reaction. If you're closer to olimar you roll if you're farther you spot dodge. If you do it on reaction, dedede's roll is faster than the grab and his spot dodge lasts longer than the grab. This is an amazing flaw in Olimar's gameplay against dedede because it takes out his only close quarters advantage. If you elect to spot dodge, ftilt immediately! If you roll around him, grab him and throw him off, another gimping opportunity.
Where we win the matchup, offstage:
Now is the most important part and I'm sure you're already good at it so i'll just throw out some advice. If oli recovers low your only choice it to try to bair his jump, but what we're actually hoping for is that olimar recovers high, which most do. Jump BELOW olimar and uair! Olimar CANNOT whistle the whole uair and the last hit will send him further offstage allowing you to grab the edge and end the stock. Practice spacing the up air right so you can send him in the correct direction, you obviously don't want to send him back to the stage. Just takes some practice, you'll get it.
Summary:
Ignore the pikmin, let them grab your body so that olimar doesn't have his weapons. Powershield the purple throw. Ftilt ftilt ftilt. The whole matchup is ftilt, it's the best move you have and it outranges every single one of olimar's moves and it kills any pikmin he tries to throw
55:45 Dedede