Usually when someone says an MU is even (or any ratio really) it's implied that the players are of roughly equal skill and match-up knowledge. MU knowledge is also a two-way street. Corrin can certainly improve their MU odds by figuring out the MU, but that also applies to the other character(s) as well. And I doubt that the playerbase at large has a super good grasp on how to best combat Corrin.
Zoners ironically struggle with themselves because they tend to lack the attributes that are good for dealing with zoners. I honestly think that the archetype of zoner is ultimately doomed to fail in this game really.
Now, I don't think that Corrin is a bad character, because they're not (note: "not bad"=/=top tier), it's just that I'm skeptical of claims that anyone is top tier without sufficient data to believe so. 9 months might seem like a long enough time to say for sure that yes, X character is indeed Y tier, but it's kind of not. When Roy first dropped, a lot of people believed he was high tier, but eventually everyone realized that he is certainly not high-tier level good. Opinions on Cloud initially ranged from barely mid tier to top 2. Bayonetta is different though because the character has garnered far more exposure in this time, has a larger high-level playerbase, and it's obvious that the combination of Witch Twist+ABK (Witch Time is just icing on the cake really) is absurd enough to warrant her a spot in top tier. Or to use a non-DLC example, it was not until recently that I felt could both say with some level of confidence and articulate well enough that Robin is indeed nothing more than a mid tier and is likely to not ever be more than a mid tier, and that character's been out for 2 years now. When I asked "why is Corrin top tier", I was kind of wanting something more in-depth and specific and not nebulous claims like "goes even with/beats everyone in top tier except these 2" or "[insert list of Corrin's strengths here] (simply listing a character's strengths is generally a poor argument, as you can make anyone look better than they are by doing so, even trash like Dorf or Jiggs). I'm also just highly skeptical of anyone in this game with overall poor mobility doing truly well.
tl;dr: I don't believe the claim that Corrin is top tier because I think we should have more information and meta development before we do make that claim.
Corrin, and zoners in general, are good at creating walls, that are virtually impassable. Now whether or not these characters do well depends on whether or not they're able to create the wall.
I would say that the concept now is that zoners beat boxers, boxers beat defensive(long range projectile heavy), and defensive beats zoners. Zoners beat boxers by zoning them out, and preventing them from getting in. Boxers close the gap between defensive and them, and defensive usually don't have good boxing options and are generally considered to be slow. Defensive beats zoners because they don't have to approach zoners, which nullifies the sobers concept of a wall.
Now, in a game as diverse as sm4sh, this won't always apply, as there are few just defensive and few just boxer.
In the current meta game, zoners are definitely good, but it heavily relies on the individuals ability to create a wall, and guess an opponents options to breach said wall. I would say zoners, are becoming far and few, being replaced by super defensive or super aggressive players.
To answer your question, Corrin is good because she has a lot of room in terms of play style, as wells as a lot of really good tools. She's not S tier, nor even top A tier. She's a hidden boss in the A tier, a top B tier that can sometimes be really really good. She can be compared to Yoshi, since one of the reasons both are so good is because there are so few players with the MU experience.