You could say Corrin is not relevant in the current meta and you would be right, specially on the major/national scene. On the regional level, we've got some people like Biggad (Germany), Vapour (Nova Scotia, iirc), Nom (Michigan) and YOC (Japan) bringing in some respectable results, and of course Cosmos dominating the DFW area for a while now, but nothing special overall compared to a lot of other characters.
Ryuga solo mains Corrin as mentioned above, and while his previous record (even with Ike) would make me see him as the best Corrin rep at the moment, he's kinda been MIA lately so that title fairly belongs to Cosmos. Ryuga also isn't attending the big summer tournaments so it's unlikely we'll be seeing stellar Corrin results anytime soon.
A lot of people value her as a counterpick/secondary, but even then I don't see her being used as much as before 1.1.5 where players like 6WX, Larry, Regi and a slew of others would use Corrin and do well, but promptly never touched the character again around the time the patch hit. Now, I'm of the opinion that the nerfs didn't affect the character much; if you were good with Corrin in 1.1.4, then you'll still be good in 1.1.5 onward. But imo, the fact that the nerfs
did happen made a lot of people realize that they didn't actually enjoy Corrin or that she wasn't as good as they thought from the get-go, and the patch just so happened to be a convenient time to jump ship. (Imho this same effect partly explains why Greninja fell off the face of the planet for a long time after Wii U release, but his nerfs were actually substantial compared to Corrin's)
There still are a few high-level players using Corrin as a secondary, like Remzi, Ryo and Esam, but all three perform notably better with their actual mains. Ryo also secondaries the entire FE crew + some others, so it doesn't really help Corrin's case, and Esam...well, his secondaries are notorious for being sub-par, even if they do take games from time to time. There's also our resident subforum mod
@Raziek who secondaries Corrin, but if his GOML run is anything to go by, his Robin is by and large the one doing work
also please don't leave us <3.
I personally don't believe that being competitively irrelevant inherently equals being a bad character, and Corrin has some special circumstances holding her back as already explained in this thread. Being relatively new and a not-as-obviously-busted-out-of-the-gate DLC gives high-level players little reason to drop their mains whom they've been working on for over a year now to learn a new character from scratch, with completely unique tools and a distinct gameplan. With no top-level talent backing her, Corrin's already underdeveloped metagame further stagnates, thus creating a vicious loop. It's up to new blood to prove her worth, and they won't be seeing national-scale success overnight so for now we gotta buckle up, git gud and see where this ride takes us.
TL;DR: Competitive Corrin IS [mostly] dead at the moment, but that doesn't mean that we should stop trying. If anything, Corrin being a relatively unknown character gives us some edge in terms of MU inexperience on the opponent's end :^)