There is no reason, granted you are cleared by a physician, that heavy people shouldn't do yoga. If a person has high blood pressure, it is especially important to check with a doctor because some medicines have adverse effects, like dizziness or decreased heart rate, and some poses, like headstands, may not be recommended.
I think yoga can help in a variety of ways. In addition to improving flexibility, which is often overlooked, it can also help reduce stress. This, in turn, can help battle emotional and stress eating if that is an issue. It also helps you make the mind, body, breath connections that will be beneficial in all areas of fitness.
I think you need to be realistic about calorie burn, which is what you really need to lose weight. Although it can easily be incorporated into a fitness regimen, more emphasis should be placed on cardio and strength training if fat loss is your goal.