You should definitely tell her how you feel.
If one of her motivations is that she believes that you'll find her more attractive afterwards and she later discovers that your opinion of her attractiveness hasn't changed then she might feel a bit disappointed or possibly even decieved if you haven't told her otherwise.
I'm not implying that not telling her is a form of deception, I'm just saying that it's possible that she might feel that way, especially if it's a bigger motivational factor than she's letting on.
Sometimes people get ideas in their heads about how people will react to something that turn out to be drastically different from how the person would actually react and it can be very disheartening when that happens, especially if a person has gone to a lot of effort.
All in all though, it's better to be sure and being able to be open and frank about your feelings is important in a relationship.
When she next brings it up, sit down with her and have an open and frank discussion about it.
Ask her if she's losing weight purely for health reasons, or of part of her motivation for losing weight is because she thinks she'll be more attractive.
Whatever she replies, tell her that her weight loss won't make you find her any more or any less attractive than she already is. Then reassure her that you will continue supporting her, but you just wanted to make sure she understands how you feel about it because you don't want her to be under the false impression that her losing weight will affect your opinion of her attractiveness.
After you've discussed it with her, go ahead with whatever she decides, let your mind be at ease and don't bring it up again unless she wants to discuss it further.
Generally speaking, honesty is the best policy.
This is an especially difficult subject because for many years the world has conflated skinnyness with attractiveness, and the media has made sure to reinforce that notion as much as possible.
As the saying goes though, beauty is in the eye of the beholder - everyone has different tastes in what they find attractive (e.g. skinny, fat or muscular; men or women; tattoos or no tattoos; dark or pale; make-up or no make-up). For every person that finds someone attractive, there's another who doesn't.
Hence I personally think weight loss should only be done for health reasons or if a person truly thinks that it makes them look better in their own opinion, and not because of the opinions of others. Either way, for some people it can be a slippery slope, and it shouldn't be done on a whim.