[DanCooper]

TV Movies don't have an MPAA rating, but shouldn't that field be populated with something equivalent like the TV Parental Guidance rating (TV-G, TV-MA, etc)?

And what is the point of the Certifications field? It's always blank.

 MPAA is a US-only rating like "Rated R for voilence" and only available for movies, certifications are shorter and available for most of all countries, like "Germany:18" or "USA:R", and also available for tv shows. Only IMDB provides MPAA values, all other scrapers has only certifications.
Kodi reads only <mpaa> in NFO. For that reason you can set that the certifications will be used as a MPAA value. Certifications can be filtered to one country only in the settings.