To use iNAP successfully, you must breathe through your nose. Many mouth breathers are able to breathe through their nose, but based on early habits, have grown to be predominant mouth breathers.
If you are able to breathe at night through your nose and can use a chin strap or tape to help train you to keep your mouth closed, it is possible that over time, you would be able to use iNAP.
There are many articles and resources that discuss the benefits of nasal breathing. Also, it may be good to speak with an ENT who can assess your nose and airway and possibly offer suggestions to improve your nasal breathing.