The forms are useful only if you are using the Apps layer in Activiti 6. If you are doing external rendering, why do you need to export the forms unless you trying to implement the form rendering logic in your application based on your form model in Activiti 6?
ok, I thought you were looking to use an external form in your application. If you are looking to render the form that you model in Activiti 6 in your application, you will have to implement that logic in your application.
Btw, if you are using or planning to use the enterprise version of Activiti (Alfresco Process Services), you get more advanced form building features and also a framework called Application Development Framework (based on Angular 5 now) for rapidly developing custom UI applications.