Unfortunately, all kinds of potentially good stages are ruined by the presence of an overly strong hazard or whatever else.
I can't believe how much of a missed opportunity Omega Mode is. Rather than just making every stage FD, it should just make it so most stages stay the same but have their hazards removed or are slightly altered to be more competitive. For example, if the Omega Form of Reset Bomb Forest was just the first transformation, it'd be legal, no question. Omega Wily's Castle should just be the same thing but without Yellow Devil. It would also no doubt be legal. Omega Magicant has no Flying Man, Rainbow Road has no cars, etc.
Anyway, back on topic, I'm becoming much more convinced that Yoshi is indeed high tier, mostly because it's becoming clear that a lot of the characters who initially looked quite good have some holes in their game, while Yoshi remains a pretty complete character. That being said, I think Yoshi has some issues vs the top tier characters. Sheik is just straight up better than Yoshi in almost every respect (though she's straight up better than virtually everyone, so he's not alone than this). Her combos are way better, her recovery is much more versatile, and she's faster and better at neutral. Those needles are a problem for Yoshi too. He has no trouble killing her if he can land those hits, but it's hard to win the damage race against her. Little Mac I think is just a straight up bad matchup. Yoshi's kill potential is almost 100% vertical and Little Mac appears to have the best resilience to vertical KOs in the entire game. He can gimp Mac if he can get him offstage, but he has very few tools for doing this, and this makes it easier for Mac to counter those options. And of course there's KO punch, which makes Yoshi's normally fantastic survivability a moot point if it connects. The one top tier character I don't fear at all is Rosalina, Yoshi seems pretty decent at dealing with her, his moves are good at killing Luma while remaining fairly safe, and she is one of the floatiest characters in the game so she dies off the top really early.