Honestly, it would depend on other factors.
My ex husband was slim when I first knew him. He was quite an energetic person who had a lot of hobbies and did stuff. By the time we got together, he'd put on a bit of weight but nothing too much. Over the years he fell into obesity.
I was concerned for his health because he wasn't a fat and fit person. Everything became too much effort or painful to do and he was plagued by weight/lifestyle related health issues. Our lives became very small because of it. It was that which made him become unattractive. He developed type 2 diabetes which he didn't manage properly and so became even more ill. My life was severely impacted by his choices. All he wanted to do in the end was sit in front of the TV eating.
He has since lost the weight and got his diabetes under control. He now does stuff again.
So it depends. If a great weightloss has improved someone's vitality, quality of life, overall health etc then I can see how that would make them a more attractive person overall.
If, however, the weightloss was a size 14 to a 10, that's very superficial and suggests its purely motivated by appearance.
My boyfriend now is overweight. He was always the 'fat kid' (apparently). Doesn't bother me in the slightest because it doesn't have any impact on him or our day to day lives.
I've known people who were fat, fit, and generally took care of themselves and others who were lazy, slovenly and complained constantly about ailments. The former were attractive and their weight made no difference to who they were. The latter were, frankly, hard work.