Probably depends how much they let themselves go, I'm quite health conscious myself so I would prefer a partner who thinks likewise but obviously there are other factors to consider. If you actually intended to be with them for the long term then maybe it would be best to try to resolve why they were actually 'letting themselves go', it would be pretty harsh just to straight up finish the relationship if you otherwise got on well (within limits obviously, I could never be in a relationship with an obese person for example).