I disagree. K.Rool is the main villian of Nintendo's 4th best selling franchise and the 2nd or 3rd most reocurring villian Nintendo has behind Bowser and Dedede. IMO he's been WELL deserved and overdue for Smash since Melee. It's a shame us fans of him have to fight/kampaign as hard as we do to get Nintendo to know we enjoy this character and want him to return to Smash and his own franchise (DKC). While I agree Un-represented franchises like Golden Sun, Sin & Punishment, Chibi-Robo, Splatoon, Rhythm Heaven, and Advance Wars should get some representation too. I don't believe they deserve to get in before K.Rool does considering he's a villian something Smash Bros is severely lacking in, DK content something Smash 4 is also lacking in, A new heavyweight something Smash 4 lacks in as well and just overall what a great and iconic character he is.
As to your "Mario character" point. While DK and crew cross paths with Mario a lot I view DK as its own franchise. Remember Mario was a spin-off of DK not the other way around so if anything it's TOO many DK characters if you want to view it from that point of view. lol Also King K.Rool himself has only appeared in one Mario spin-off game (Super Sluggers). Know what other characters appeared in a Mario spin-off? Link, Villager, and Isabelle in MK8. So I guess they're Mario universe characters now too right? lol or what about the final fantasy/dragon quest characters in Mario Hoops, Sports Mix, and Fortune Street? What about Sonic and friends in the olympic games series? lol Obviously i'm exaggerating but you get my point.
That being said I'm a supporter of the following characters: Chorus Kids, Krystal, Wolf, Ridley (never happening unfortunately), Medusa, Dixie Kong, Black Shadow, Anthony Higgs (yes i'm serious he can take Snakes old moveset), Inkling, Chibi-Robo, DJ Octavio, Cranky Kong, Issac, Sylux, Captain Toad, Isa/Kaichi (new ice climbers for Smash 5?), Impa (Hyrule Warriors form), Bandana Dee, and of course King K.Rool and out of this list King K.Rool, Ridley, and Wolf are the last remaining "musts" in my book to really make this the ultimate Nintendo All-Stars game IMO.