The MU is difficult because Dedede has no reliable way of getting inside of ICs and separating them that isn't extremely telegraphed or risky.
Like Blizzard Wall is generally an overrated tactic vs most characters but its's pretty useful for keeping Dedede out. We CAN beat it with D/F Smash but time it wrong and well...yeah.
Be super wary of Ice Block frame traps, especially when you land. Don't allow yourself to be caught up by Nana doing an Ice Block just to get grabbed by Popo.
In this MU, you have to take a lot more risks than you may be accustomed to doing. Grabbing ICs works pretty well if you can land it. F-Throw is awesome in this MU because it's very quick and gets out IC out of there very easily. We **** solo ICs.
Swallow is also useful but if you Swallow Nana it can get you punished. It's a move you'll want to use sparingly and only when you're sure you'll get something from it.
Look out for Up-Air as well. Naturally, people want to try to stay in the air for approaches but Dedede's air speed is ****. So it's not that effective. Especially when ICs broken *** Up-Air. Our Dair CAN separate them, but its telegraphed. You have to be creative when you use that move.
You just have to constantly mix up your approaches until you find opening. You can't stay on platforms the whole match, and you don't want to stay ground for long period of time either. You have to stay on your toes. I also just take small punishes with F-Tilt here and there. You can hit Nana with it, fairly easily. Eventually, when Nana is at a significantly higher percent that Popo, you can take advantage of that.
PAY ATTENTION TO WHAT YOU HIT NANA WITH. TRACK HER PERCENT MENTALLY IF YOU CAN.
It's a really frustrating and draining MU.