Heya, friend! Thanks for dropping by our little corner of the internet and sharing your story.
I'm a fan of DeDeDe, even though my history with Kirby games amounts to a couple days with the Gameboy original and a couple days with Super Deluxe. Kirby, Meta Knight, D3 -- I love 'em all. Fun characters with lots of pinache. Ever since someone on Reddit showed me a Girthquake video (I subscribed!), I've fallen back in love with D3 again.
All that to say, I really, really, really hope Mr. Sakurai & team finally buff the remaining low tiers in this (potentially final) upcoming patch! It's like you said: Mewtwo is now a total blast to play! He went from a bottom tier (some said WORSE than Melee) to a viable mid tier pick in one fell swoop.
Sure, we have these characters like Olimar and Duck Hunt who have these difficult play styles that people agree have potential, but get ignored... jury's out on what exactly they could need because not enough skilled players use them. But... then you have characters like Zelda and DeDeDe who you have to be a genius to pull off anything impressive with at top level play. With these kinds of characters, you need the Mewtwo treatment: make all their best options better; improve movement options; don't give them a Hoo-Haa, but make what they have work properly; make them fun to play again.
Not every character can be top tier with a cast of 58. Someone has to be worst. But, "worst" doesn't have to resemble Smash 4 Zelda, whose Nyaru's Love doesn't even function properly.
I've heard it suggested that if DeDeDe's jab 2 was improved (faster FAF, better angle, more hitstun: one or more of those), many of his problems would be mitigated. Not all, but several. It'd turn D3 into a combo machine and possibly give him a kill confirm as well.