Corrin's not a top 20 character by far; the lack of a combo throw, low mobility, average recovery (unless near the wall) and sometimes lack of kill moves are a hindrance, but his good kill moves, good combo ability and great spacing ability all but make up for it. Corrin's a very simple character, and it wouldn't take much for an opponent to find out their weaknesses and exploit them to the highest degree. Corrin, however, does have an anti-combo move by the name of Counter Surge, which is only reliable if your opponent commits and goes for a powerful attack or an attack very close to the ceiling, or, you know, your opponent is Ganondorf. Dragon Shot's not a bad move either, it's actually probably one of the best moves for getting quick damage, because all you need is an uncharged projectile to lead up into a fully charged bite.
In terms of placement, I got to put Corrin around top 30, around 27th to 28th. Firstly, Corrin's not that good of a character at all due to the poor mobility and lack of a combo throw, but he does at least have some good combos and kill moves; though some of his kill moves do have a bit of endlag to them, making them very punishable up close. Also, I've not seen many results where Corrin's made top 8 or even top 3, so for me, Corrin's been slipping down and down for me in terms of tiers.