I emphatically disagree with you here, but you must have known I would. First off, I don't think either of her Dragon Fang skills are gimmicky. Dragon Fang Shot is a projectile that also doubles as a melee attack. It's simple in application, fun to use, and its potential for set ups is extremely high. Dragon Lunge is like
the move for Corrin. You can control space, build damage, KOing, even use it to recover... it's pretty much the perfect move. Up B and Counter are whatever, but their neutral and Side Bs collectively have some of the most depth in the entire game. I disagree with you on transformation attacks being glorified sword attacks. Back Air and Down Air have unique properties, Fsmash is like the
king of forward smashes, down smash has massive range on both sides, and unlike your dime a dozen Link-style down smash, it hits on both sides instantly. I already covered neutral and Side B.
There's another big aspect to Corrin that you overlooked, and that's
her combo game. Most sword wielding characters can't consistently string their hits together, but Corrin can, and it's one of her defining strengths. Like, if we're talking about bringing things to the table, Corrin is probably the most "solidly" designed swordsmen after Cloud (who is loaded with stupid nonsense).
I know you're biased (i'm biased too, lol) but I don't think you're giving the character enough credit. Say what you will about the concept, her character ended up being pretty damn great in practice. Especially considering Cloud and Bayonetta are (still) rife with nonsense plaguing their moveset, and Ryu doesn't translate that well into Smash Bros imo. I honestly think they stack up extremely well relative to the entire roster.
Making all of her sword attacks into multi-hit moves would have been a disaster since multi-hit moves can be SDI'd. Their combo game, projectile and Dragon Fang moves do a good enough job of setting her apart from other swordsmen. My only real grievance I have with Corrin in Smash is the Omega Yato's sound effects. It makes this incredible
SMASH sound when it hits something in Fire Emblem:Fates, but it sounds pretty feeble in Smash.