In Jiggles's case, she's a good character with a unique game plan and is pretty much the antithesis of Little Mac. Here's some problems: without edge-hogging, she can't really gimp like she used to where if she hits you off and grabs the ledge, you were down. It's a universal problem, but Jiggles used the ledge like it was her game. Now it doesn't exist so... Jiggles also exploited everyone's recovery and in a game where recovering is just easier because nobody really does anything off-stage, many recoveries are just plain too good, and that you can't get edge-hogged, Jiggles can't really do much that other edgeguarders can also do and sometimes even better since they can get their opponents off the stage much more easily. Another thing might be aerials and landing lag changes and shield changes since if Jiggles could push you off the ledge with any move, that would be terrifying as it would for other characters e.g. Falco could potentially rapid jab shield push you to Nair confirm and it Ryu might be able to light Dtilt lock to heavy Dtilt shield push and Dair.
For Marth, at least he (and Lucina) have been changed. If anything, Marth's the character changed to most while moving and looking similar to past games. He's much more spacing-orientated than setup and combo-orientated like in Melee and Brawl. They kind of made Marth go from a swashbuckler to a fencer while making Roy the swashbuckler. So, in terms of Assassin's Creed's combat system evolution, Melee and Brawl Marth would be like Brotherhood, III, IV: Black Flag, and Syndicate's? where you could pressure quickly and attack without much of an issue while Smash 4 Marth is more like I, II, and Unity where Marth can't attack, attack, and attack, but must play more defensive and cautious. In Smash's case, the community doesn't really like defensive play, so they don't like Marth in Smash 4. Maybe they do now since he does feel like what he's doing works unlike in the beginning where it sort of worked.
Falco's just no translations in all games. They did keep this in all the games, though: he's vertical. They did keep his vertical-orientated combos, but it does feel silly when that's the idea around him. Melee Falco was a vertical combo-heavy character who moved slower than Fox, but hit as fast and harder than Fox, has a projectile that stuns, and has a recovery that's just butt. Brawl? FANTASTIC! Let's give him the most broken projectile in fighting game history so Falco is still vertical combo-heavy, still hits hard and fast, ends up with a non-evolved Up Special and arguably, Side Special, unlike Ganondorf, Melee Luigi, Smash 4 Roy, and even Toon Link, but is now heavily spammy and projectile-wall-y. In Smash 4? Still hits fast and even harder, still is vertical combo-heavy with 1.0.8 rounding it to just combo-heavy and is now good at edgeguarding with still ****-poor recovery. The idea would be like Sonic = fast and Sonic ends up being a fast, but weak-hitting character in Brawl, Ganondorf levels of power, but still fast a hell in Smash 4, and Jigglypuff, but a jet in Smash 5.
Smash 4 Mewtwo's similar to Melee Mewtwo, except they made him really, really light. Why? I don't know. Hell, if he had the same weight, he might still be where he is today. Is that okay? Sure, since he's functional despite his current weight making it feel really crappy when he dies pretty earlier instead of early, but not as early like Zelda. For Mewtwo, his design, well, almost everything, works. That's something you can't really say for Zelda whose design in any setting is difficult to play. In any game, sweet-spots are cool, but when one half of your moveset is comprised of sweet-spots with really sour sour-spots, it's really bad. Marth and Roy's sweet-spot and sour-spots balance each other out. They can land sweet-spots to gain massive rewards while landing sour-spots aren't that bad, but not what they really wanted if they want to kill or launch people. In Zelda's case? Fair and Bair's rewards are good, but they aren't that kind with how the sweet-spots are and the sour-spots are almost Little Mac Nair or pre-1.1.0 Samus jab 1 levels of punishment.
Here's the thing: you can "fix" Zelda without overhauling her, but that comes with a consequence as with everything. Imagine this: let's weaken her Fair and Bair. You're probably screaming, "WHY!?", but hear me out. Give her Falco Bairs for her Fair and Bair; translation: weaken the sweet-spots so they're not stupidly strong and make the sour-spots so they're not stupidly weak. This also includes lowering her landing lag and changing her auto-cancel windows. Cool! Zelda can actually approach with aerials now... except... We just created a monster. You can fine-tune this even so it's not as crazy - Fair is more like Lucas or Wolf's so she won't have this stupidly strong option in front of her -, but thinking about it, Zelda would end up having really good options in the air to cover her sides. Is that what we want? Are we going to be fine with it? I mean, she's still slow, so she can't run or walk around like Falco or Wolf and start walling people out and she's much taller than Dr. Mario, so her attempts at walling might end up having her pretty little head getting clipped. She also would still have issues with her Din's Fire and Phantom Slash - her mid- and long-range game. Also, this does take the charm away from her as this character with a stupidly precise and stupidly strong Fair and Bair.
The idea with Zelda is she's the mage archetype. Other "mages" in this series includes Lucas, Mewtwo, Ness, Palutena, Robin, and Rosalina. Lucas and Ness are kind of like mages where their "spells" take time to "cast", but to make up for this, they're like monks who have good options close up. So, maybe paladins, but without weapons and armor or perhaps "magical monks". Mewtwo is like a traditional mage in Smash 4 who can dish out a ton of damage, but can't take damage - Black Mages are like this for Final Fantasy fans. Palutena's a mage in a sense of her close-up options aren't "good" since they're slow, but her mage-y-ness comes from her Specials which being all different makes her like a mage who has an arsenal of spells. Unfortunately, because of how Specials and customs work, she can't access them all. Robin's the spellsword or Red Mage who's capable of both magic and swordsmanship letting her fight at whatever range she pleases. This does come at a cost of her not being able to specialize in one role unlike Ike who specializes in close-range combat with his sword or Mega Man, like an archer, specializes in ranged combat. Finally, Rosalina's the puppeteer, summoner, and necromancer-type of mage who as you guessed, makes use of summons to fight for her. So, what about Zelda? Zelda still fits a mage since her Specials are like the AOE-type spells mages use... except they're really underwhelming and even weak... Din's Fire could be much weaker by losing a sweet-spot, but do constant damage in a large area kind. Done right, even in FFA, it wouldn't be that abusive of a move and Phantom Slash should have been an actual summon of a freaking spiked wall instead of a telegraphed projectile. Those are "minor" issues to a easily recognized design... and then you look at her aerials. What's with the heavily precise Fair and Bair? I can't think of any mage who uses heavily precise spells. In most cases, it's giant explosion this, summon that, and the most "precise" would still be something like telekinesis to manipulate fragile things - use the Force to create your Lightsa, Zelda... Yes, Zelda, I know the Force isn't "magic", but still - or possession which you could end up launching a ball of goop which wouldn't be that fine-tuned like sewing.