"She has lost weight."
Translation:Hon har gått ner i vikt.
No, it implies trying to lose weight, but not necessarily succeeding.
In fact I've known several people who put on weight from their bantning, even though that was not their intention.
However I think if you use the word about diminishing someone/something else than yourself, it does imply being successful.
I doubt anyone would say Vi har bantat katten 'We made the cat lose weight' if the diet failed. And certainly in abstract meanings when people say things like Vi har bantat innehållet 'We have reduced the content', it's also implied that you were successful. About a person though, I'd say Hon har bantat means She has tried to lose weight or She's been on a diet.