Amelia is, no contest, better than Wendy. She's in an easier game that allows exp grinding. And in any environment that doesn't allow for grinding or abuse, they're equally pointless anyway. So Amelia at least can
do something. Trying to "train up" anybody in FE6 is a pain and an ordeal in Hard. Hell, you need Old Marcus's help to train the good
characters. Wendy dies to archers.
The other characters mentioned? Eh, I don't think they're even remotely the worst:
- Ardan's durable and his combat is actually okay. He may be one of the worst units practically speaking, but it's not so much because he sucks himself as because he can't get to any fights where he'd be useful. That's a map design issue, not his fault. Plus he can be a dad and pass on Ambush and a good STR growth, so there's that at least.
- Sophia is retardedly underleveled, but at least in theory once trained she could be better than Wendy. It's basically equally impossible though. Still, Sophia finds a promo item and Wendy breaks a wall, so Sophia wins there.
- Lyre is nowhere near the worst unit in the series. Bad in Radiant Dawn? Sure. Her bases are not great. But she can be fixed, at least, if you wanted to use her. Basically you slap any stat booster on Lyre and it counts double when she's transformed. A single Energy Drop is +4 STR to her. Imagine how many walls Wendy could kill with +4 STR!
So yeah, Wendy is the worst. Amelia's just worthless in the context of a game that makes it pointless to bother with her, but at least you can try. Wendy is simply unusable in the context of her situation or anywhere else.