Corrin is looking like a top/high tier character to me.
It's no coincidence that Ryuga, Ryo, and Earth are focusing on Corrin and taking breaks from their normal swordsman mains. Corrin probably has the best neutral game of the swordsmen minus Cloud with Limit, and the second best disadvantage too, right after Meta Knight.
You can go on all day long about her mobility problems, and it's true that some of her mobility parameters leave something to be desired, but neutral is all about the bubble of space that you threaten, more than your raw mobility stats. Corrin does well in neutral for the same reason that Villager manages to do pretty well in neutral despite subpar mobility, because his effective range thanks to f-air/b-air is enough to threaten areas where others might not be able to reach even with better mobility. And for all of Sheik's awesome mobility, if she didn't have Needles to threaten at long distances and to punish rollbacks on reaction then she would just simply put not be top tier.
For Corrin, the threat of Dragon Lunge and f-smash lead people right into the jaws of reverse DFS and n-air. With even a little bit of stage control, Corrin can lock you down at tipper range for her long-range moves and then set up a vertical wall of hitboxes using things like full hop f-air -> n-air (which just covers so. much. space.). She's also got an f6 mid-range answer to shield that sets up into a kill throw at high percents and a whole bunch of other stuff at mid percents. With all of that, you'd expect a pretty bad CQC game, but actually her jab is f5 and the transitions are actually faster than Cloud's (Corrin's jab1 to jab2 FAFs are 28s and 30, while Cloud's are 30 and 33, even though his jab is a frame faster on startup).
When your attacks in neutral are so good, you don't really need top-tier movement parameters to compete with top tiers, but Corrin actually does have quite a few neat movement tricks anyway: lunge, b-reverse DFS, b-air, and randomly the fastest dash to shield in the game (7 frames, 1 frame faster than Sheik's 8, iirc). When you consider her kit of random tools that adjust her threat-bubble, you see that Corrin is actually moving like a top tier for much of the match, just usually within a smaller radius of total movement.
And compared to other non-Cloud sword characters (or characters with similar range and disjoint), her raw mobility stats are mostly normal, anyway, rather than below average. Compared to Pit, she's got higher air, walk, and fall speeds, but lower run speed. Compared to Ike, she's got higher walk and fall speeds, the same run speed, and lower air speed. Compared to Meta Knight, she's got higher walk speed, similar air and fall speed, and lower run speed. Marth is a neat exception and has her beat in run, walk, and air speed, but has a tragically bad initial dash to shield animation that makes up for it.
So with normal movement stats + additional movement options, she's actually looking more mobile than the rest, rather than less.
In disadvantage, she has tools for landing that most swordsmen wish they had, like n-air, d-air, b-reverse, b-air, Lunge, and Counter--a variety of mixups to prevent her opponent from getting followups. That Counter alone sort of just invalidates all other swordsmen's disadvantage states, at least onstage (MK is better offstage). Corrin can even choose to not use the Counter all stock long, take some hits, and then punish you for a random overextension that leads into your stock loss at some point. Consider the difference between juggling a Mario at high percent and juggling a Luigi. With Mario you have nothing to be afraid of, even if his escape options are just as fast, so you might as well swing at him. With Luigi, if you get n-aired, you might die, so very often you just won't swing. Just the threat of that Counter can sometimes save Corrin in disadvantage.
The problem with her disadvantage is that her recovery is fallible, but what swordsman's recovery isn't, besides MK? You can challenge Marth's Dolphin Slash, do silly things to Aether, lolCloudsrecovery, and Pit's is not nearly as safe as some people want you to believe. Corrin's recovery has sufficient mixups thanks again to her reverses and stalls, and in several matchups I'd actually rather have Corrin's recovery than, say, Rosalina's.
Her biggest flaw seems to be that her airspeed sometimes prevents her from getting followups in advantage, especially off of her throws. Her f-air is a combo machine, up-air kills, Dragon Lunge followups are magical when they happen, etc., but sometimes you just can't land any of it. She definitely wishes that she had the easy-bake juggling of Ike or Cloud, but given the choice of a better neutral and better disadvantage vs. better general followups in advantage, I think the decision is obvious.
She's not better than Cloud or Meta Knight (because of how crazy those two are in advantage, mostly), but I'd definitely rank Corrin at least a tier or so higher than the likes of Ike, Pit, and Marth.