if you have scrap insulbrite you want to use-go ahead- people seldom would purchase it new for mug rugs- mainly because it is expensive and scrap batt is the (norm) but if you want to use it- it's your project- use what you want= there are no 'rules' about it.
personally i make mine without batting- either with a layer of flannel or even just muslin- i like them flat---or i make them out of wool- then no center layer is needed.