#(pleasedontwarnme)
On a serious note, the only two characters I am reasonably sure will be top tier in a year are Sheik and Diddy. Rosalina and Greninja are relying right now on people not knowing the optimal way to approach the match-ups and are getting away with a ton of nonsense. I am more confident in Rosalina's long-term viability than Greninja's. ZSS has a lot of problems and I think she has been significantly overrated by everybody now, myself included. But it is possible that the GC controller will give ZSS players the precision they need to avoid trades and actually you know, use back air, which is so, so hard to use on the 3DS.
I've been trying to use her lately because of her solid Rosalina matchup, and my findings are that ZSS is really good against tall characters like Rosalina, Shulk, etc., but struggles a lot against shorter characters like Diddy Kong and Ness. Against tall characters, f-air and b-air are much easier to land and SH b-air actually pressures shields. They're also vulnerable to SH paralyzer or B-reverse paralyzer at most heights, and paralyzer to up-b is a solid kill move.
Against small characters, If they shield paralyzer, you only have spaced n-airs to hit them with, and that makes me sad. I find that other tournament players walk forward, shield, know that I won't grab or b-air or anything else, and just put themselves in my face, where their jabs and d-tilts are almost assuredly better than mine (I wish ZSS' jab were just slightly better: either packed more of a punch or had more range).
Subject of Diddy:
Maybe I'm just a bad player, but I've been winning a lot less with Diddy now that opponents know to spot dodge bananas and his monkey flip. With those two options pretty easily averted, Diddy feels a lot less threatening. I still think he's a good character, but not in the top 5 like some people were claiming.
I think Yoshi is absolutely nuts. Lives forever, combos well, kills early, has a great projectile, and has probably the best MU against the character everybody thinks is the best in the game (Rosalina).
You seem to be concerned about maining someone who ends up not being that great (judging based on your other posts about that topic and how you keep asking about the viability of Diddy/Greninja/etc.). I would say, just relax, and play someone that you like. High tier is good enough to win a national. Even mid tier is. And Diddy and others like him certainly won't fall beneath high tier.
Having said that, I do want to point out that Diddy Kong is very strong. Monkey flip is one of the strongest, most versatile moves in the game, and you can tilt/smash it. Bananas are a great pressure tool, and if your opponent is spotdodging them then you might be throwing them out too quickly. Just walk around with a banana in hand (and shield if the opponent has a good projectile like needles), and then wait for your opportunity and strike! Spotdodging is beaten by inserting pauses between your play. Shielding is beaten by monkey flip command grab. Rolling is beaten...just by its nature because if the opponent rolls away he's giving up stage control. If he rolls behind you he'll probably get hit (d-smash is good: call it).
Comparing Yoshi to Diddy on an aspect-by-aspect basis (there is some overlap between aspects here, but this is also for consolidating my thoughts in addition to presenting an argument for food for thought):
* Yoshi is really good, but I don't think that he combos all that well. Diddy's followups and strings are better: you can vector his throw followups but it isn't easy to escape taking any damage between ground, air, and monkey flip (it deserves its own category).
* Yoshi's aerial mobility and aerials are great, but compare them on a move by move basis and you'll see that the difference isn't that huge. Diddy's side-b is better than Yoshi's neutral b for use in the air. Diddy's up-air and f-air are better, while Yoshi's d-air and n-air are better (though Diddy's n-air is really good: don't underestimate this move). Yoshi's d-air in particular is kind of nuts, indeed. B-air is close because Diddy's is surprisingly fast and has a lot of reach but Yoshi probably wins this because of the multi-hit nature of the move. Egg Lay isn't always useful in the air because it goes in an arc above you (though it's useful sometimes), whereas the banana can be thrown into the air to bait a spotdodge or to launch an opponent (Battering Banana). I would give the edge to Yoshi in the air because of that mobility, but the gap isn't very big, imo.
* Specials: His down-b and neutral-b combined are about as useful as Diddy's side-b since the down-b can break shields, but Monkey Flip is seriously so awesome (it can launch opponents super far at higher percents; it has so much range and unpredictability, and it's an escape tool). Yoshi's custom egg lay 3 is a great move to finish characters with bad recoveries, though, so that is a factor. On the other hand, Battering Banana is better than eggs, though eggs are far far better than peanuts. Yoshi's side-b is kind of useless (OK, not totally useless, admittedly). Diddy's rocket is pretty cool; you can arc/angle it, or use it as a surprise attack move on stage.
The general trend of Diddy's specials is to improve his neutral game. The general trend of Yoshi's specials is to...improve his neutral game. I wouldn't give one the edge over the other.
* Ground normals: This is really difficult. Yoshi's jab is far better, but Diddy's d-tilt is good enough to often function in the capacity of a jab, and you can follow up with that monstrous f-air out of it. Up-tilt and f-tilt are unexceptional but useful for both. I'd say that Yoshi's up-smash is better than Diddy's up-smash because of the ways that Yoshi can string into it, but Diddy's f-smash has a lot of reach because he steps forward and I'd give it the edge over Yoshi's f-smash.
Yoshi's dash attack is amazing for approaching, and Diddy's is only good for launching people. But conversely, Diddy's dash-grab is much better than Yoshi's. Yoshi's grab outside of neutral-b has a lot of end lag on it due to being a tether.
* Neutral game: Yoshi walks back and forth throwing eggs and waiting for an opening to dash attack or jump in, airdodge, n-air (yes this works: that n-air comes out super quickly and has a lingering hitbox to catch the airdodge punish attempt). I think that Yoshi's neutral game is good but not great: his jab is good and quick but doesn't reach that far, so he's often relying on his dash attack to approach. He can't really grab to approach, since it's hard to run up and neutral-b (though you can aerial B-reverse it, which is a cool tactic).
Diddy Kong actually doesn't have that strong of a neutral game either: he's all about follow ups. He pulls a banana and waits for an opening to side-b or banana to grab, occasionally firing peanuts. His dash grab is really good due to his ground mobility, too, and that f-air is a monster: maximally spaced, it's difficult to punish and/or shieldgrab. Overall, I'd give the edge to Diddy in neutral. Yoshi vs. Diddy is a good example of how the neutral game pans out in general for them, and I think that Diddy Kong wins this matchup.
* Survivability: As much as I've complained about Diddy's vertical recovery in the past, with monkey flip and angled rocket blasts combined I believe that Diddy is better at recovering than Yoshi, and then when you factor in wall jumps it isn't really a contest. You can usually call Yoshi's second jump and slam him, though egg toss is now absurd for the distance it moves you. On the other hand, side-b is a great escape tool, Diddy can hitstun cancel with peanut popgun, and B-reverses into canceled popguns are as solid as ever. Yoshi is much heavier than Diddy, this is true, but at best I'd call them even in this category. It might be slightly in Yoshi's favor because n-air is a really solid escape tool from combos too. Diddy's roll is better than Yoshi's, for what it's worth, though Yoshi's shield is better for avoiding pokes.
* Kill power: I would give the edge to Diddy here. Bananas and throws are a great tool for setting up for the kill: monkey flip can actually kill, up-air kills so early and you can combo into it in so many ways, f-smash and f-air can kill, even b-throw and f-throw can kill at the edge at very high percents if nothing else works. Yoshi has to read you or do something like jab -> up-smash at a timely moment to kill. He can also up-air you or b-air edgeguard you: up-air is as potent as it ever was, but I find Diddy's up-air much easier to land in this game. It's worth noting that because Yoshi lives longer he does get more time to use Rage, but overall I think Diddy wins this one when you also factor in wall jumping and its situational usefulness as an edgeguarding tool.
* Top-Tier Matchups: A bit rapidfire without a lot of justification (this post would be too massive if I were to try to justify all of these claims: just going for a really general overview): I'm convinced that Diddy loses to Sheik and Rosalina. Needles and Luma are really effective at shutting him down, and Gravitational Pull is the nail in his banana-colored coffin. I think that he loses to Greninja, Villager, and Ness, beats ZSS, Yoshi head to head, and Sonic, and goes even with Fox and Pikachu (Pikachu because of edgeguarding).
Yoshi loses to Sheik but goes even with Rosalina (or beats her), goes even with ZSS and Greninja, beats Villager, Ness (challenges Ness' air game: beats his ground mobility), and Pikachu but loses to Diddy, Sonic, and Fox.
Seems like a decent initial spread for both, with Yoshi taking an edge overall due to that Rosalina and Ness matchup.
So it seems to my view like Yoshi is better at fighting the currently perceived top tiers, but Diddy is better as a general character from the traits identified above (meaning he's more likely to hold strong as the metagame develops). I'd call it fairly even: I don't know enough to make the overall call one way or the other, and no one else really does either, but you shouldn't be having any more trouble using Diddy than you are using Yoshi.