I dont think her line is bad. I think it's almost certain that she will start as the same old OG Aerith, that didnt know and didnt want to know about all the Planet-Cetra-Related problems, but since it's a full game experience, she as the other characters will have to go through a development throughout the game and by the end, she's probably going to take up some of the "mantle" of being the last Ancient, and how she's the only one who feels what's going on. Aerith only kinda became that right before she died tbh and they are expanding that. Newcomers might think she will be the Planet's savior, since Cloud is going crazy all over the place, and oldcomers will get that terrible feeling of dread because we know what her destiny will be like. It will play out very nice, and I'm almost sure Square knows that. The problem was the voice acting, but it's not the first time we've heard a line being said terribly in a trailer. Because if I'm honest....trailers kind of suck.
You watch the E3 trailer and then the TGS one and it almost feels like two completely different tones the game will have. The E3 one is obviously suited for a more european-american audience, and TGS is classic japanese trailer. They all have their hype music and combat action cut over, but the feeling they both give are strikingly different.
And I think that's the problems with trailers. In the end, they are nothing more than cut out, mashed up pieces of advertising that have to fill in some standards for audiences to see that and wanna buy it. Everytime I see a trailer, I remember the leaked demo, and how pretty damn decent the voice acting was, and the lines were good, and the music fit right in. You watch a trailer? You just entered a freaking lottery, where half the scenes dont fit and the voices dont match, and half your excitement exists because you really are into this game. They can be pretty good to hype up you and the game, but showcasing what's really going to be like....then I dont think that's their shoes to fit.