Hi,

if you want to completely exclude the load of this plugin during the Joomla execution this must be done in the plugin source code. For example evaluating the template loaded or a global variable set in the responsivizer plugin it's possible to know if the mobile template has been loaded and exclude the mmbackground to execute.
A better option would be ask to the developer of mmbackground to add an option to manage exclusions using a parameter based on templates.

Greetings,
John