You're wildly overstating - not by a little, but by orders of magnitude - how extensive parody protection is. Porn companies have, in fact, lost lawsuits over porn parodies. If you made an entire movie featuring Winne the Pooh, there is no way, no matter how different the overall plot is from typical Winnie the Pooh stories, nor how sardonic you made it, that it would be covered by the parody exception. One element of fair use (parody, in this case) "the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes." Making use of a character for an entire movie is not going to fly. This is parody. Your idea of a feature length film centered on copyrighted characters would only fall under parody protections if you made the Mario and Bowser characters so different from the originals that it wasn't worth making at all.