I don’t want to cast too much shade on this idea but:

• Real housing needs much, much more plumbing and electrical infrastructure than malls provide
• Giant indoor atriums are very expensive to heat/cool in most climates
• City councils in the suburban areas where most of these malls are located are notoriously reluctant to add large housing stock

As much a I like this idea, I think it would be better to address these first two points by tearing down and starting over.