While there is some debate as to whether they work or not, it largely depends on what you are hoping to accomplish by wearing one.
Some women tend to think that these are meant simply to help "shrink" your tummy faster after giving birth. If that is your main reasoning for using one, then I would say, no, it probably doesn't work.
But a true postpartum girdle should offer support for your back and abdominal area, and is really considered a medical compression garment, which are used after many types of surgeries.
Besides, you aren't meant to wear them long term, so I doubt that your muscles become lazy after 6-8 weeks of wearing one.