Who are those "several high tiers" that people claim Corrin is good against? I would imagine she's about even with Cloud, Mewtwo and Rosalina - which is already pretty good, mind you - but that's all that seams feasible to me.
Ness is another one. Not a common high tier, but still a high tier currently. And being a good option against Cloud and Mewtwo is pretty major like you said. Don't know about Ryu, I ain't going to pretend I'm good at figuring out who does and doesn't work well against him, lol.
Ike's had on and off good results since at least MLG (where Ryo got top 8). The "on and off" factor is annoying/causes Ike to fall off of the radar at times, somebody already posted our johns for that issue. But considering Ike can still place top 8/just outside top 8 at majors like GOML and MLG despite those johns is pretty telling.
Currently its Ike > Marth > Corrin > (Lucina + Robin) > Roy. I'm not super confident in what Lucina can and can't do so I'm a bit fuzzy on her placement overall. Robin is most likely above her but there's some fudge factor there. Marth is
really close to Ike at this point. He's making a lot of secondary appearances (more than Corrin), and his solo mains are starting to get there.
PK Gaming
Ike's top tier spread outside of Diddy/Sheik (Maybe Fox as well, though that MU is more volatile to the extreme than anything. Both characters can kill each other within about 30 seconds of the match starting) is all within the 45-55 to 55-45 range, with Sonic being an arguable 6-4. Its not amazing to be sure, takes a lot of effort, but its something that's workable for placing well at majors. Hence MLG and GOML results. Those Diddy/Sheik issues hurt for sure, but being able to take on Cloud, ZSS, Sonic, Rosalina all fairly comfortably is fairly large as well.
I think what's more telling, is that despite that high tier MU range, Ike doesn't qualify for high tier with the current standards. If Ike doesn't make it, how on earth is Corrin supposed to qualify with a MU spread at that level really close to Ike's, but nowhere near the same level of important results?
Hero_2_All
Corrin's regional results are not at a high tier level. They fit
perfectly into the mid tier range of "one or two regions have a good one they have to deal with". You also can't get tossed into high tier based on theory. You need results to back it up. Corrin doesn't have it, and he's not building any steam.
No results, no high tier. No "new character" johns.