Oh I know they aren't, but they are very strict when it comes to their IPs.
They're very strict but not always (in fact, rarely) for the right reasons. In recent years, there have been a number of anecdotes on Nintendo's apparent mandates on character and enemy design that stifles creativity in favor of brand recognition, even when the product being released is a lifeless, corporate entity with little to no personality, which is more or less what the decision to cast Chris Pratt is being criticized as, and what this movie ultimately is demonstrating to be in the eyes of many people.
Compare, for example, how protective Disney is of Mickey Mouse, to the point where they wouldn't even let the writers of the newest incarnation of DuckTales use him, forcing them to instead use an expy (a watermelon with Mickey Mouse's general shape and voice, as a reference to
Cast Away). Meanwhile, incredibly subpar CGI cartoons with an abundance of animation errors and clipping issues astoundingly manage to get the green light.
Nintendo isn't nearly as bad as Disney in this regard, but I think being anything more than cautiously optimistic about something because of Nintendo's history of being a helicopter parent for their IP's sort of flies in the face of their actual creative output, and especially flies in the face of the fact that we can hear with our own ears how ill-fitting Chris Pratt sounds in the role.
To me, the problem isn't whether or not the film looks good or bad, it's whether or not it's the type of film that I or anyone wanted from the Mario series in the first place. When there are YouTube videos made using Source Film Maker that can more closely adhere to the style and tonality of the series, I struggle to think of a legitimate reason (that isn't muddied with corporate jargon and legalese, which I don't really regard as ironclad defenses) that Nintendo themselves couldn't release one themselves, even without the involvement of a major studio like Illumination.