Yes, I personally used a post-partum belt for around 5 weeks and it helped get my stomach muscles tighter. It also supported my back since right after delivery, I felt my lower back was not quite sturdy.
Even my mother said that tying a soft dupatta helps -- one of her sisters used it back in her time and she's still the slimmest and fittest among her other sisters.
However, I did learn that a belt alone won't help lose all the flab, you might have sagged skin but to truly tone those muscles, you'd need to work out once your doc OKs exercising.
SandhyaYadav