The clear response to this, of course, is that the character roster should try and please as many Smash fans as possible. Someone will always be upset that their character isn't in, but by adding in the fan favorites, the majority will hopefully remain content. And I have to disagree about the Mario and Pokemon assertion; if the roster was dictated by sales, and sales are considered interchangeable with series character popularity, then maybe, but in general, you'll have maybe one Mario or Pokemon frontrunner (Mewtwo, no real Mario frontrunner this time) who stands out as the best choice.
Also, the fallacy "Durrr, if fans got what they wanted, Shrek and Naruto would be in" implies that every fan request would simply be copy-pasted into the game, no questions asked. They'd still have to pass the sniff test for quality and common sense, but for the most part, that wouldn't be a problem for characters like Ridley, K. Rool, Takamaru, etc.