If it is just because of her looks yes. If the only reason is concern for her health then no. Don't push or critize if she is trying just encourage. If you think she has lost even just 5 pounds say something nice about it.

If you are asking her because you don't like the way she looks then you should ask yourself what is it about her that you really love and can you love her for how she is on the outside just as much as on the inside. If you don't find yourself physical attracted to her as she is now may be you should not be with her.

How do you men feel about larger women? Would you date a woman who weighed more than you?
What would you do if a plus size lady approached you and started flirting?