Currently we support only basic HTML editing but not an advanced drag and drop editor. We have plans to implement an advanced drag and drop editor. We will let you know when this feature is ready.
If you are looking for a sophisticated HTML designs, we strongly recommend using a third party HTML design tools like beefree.io or take the help of a HTML design expert to design the layout you wanted and load that HTML into Salesforce Email Templates and reuse them through MassMailer.