Yeah, K. Rool is pretty much a lock. Nothing can knock him off his pedestal as the most wanted and popular Nintendo-owned character not in Smash from one of their longest and largest franchises ever.
And while I think Mewtwo's chances are very high, I wouldn't say he's a lock. A shoe-in definitely, but a lock, no. And no, I don't think that comment means much, because that seems to be his generic answer he has for everything. Plus remember guys, Sakurai is the troll among trolls. He gives us one character we thought was unlikely, the only 3rd party newcomer we thought was a shoe-in, and a character no one saw coming. Who knows what he'll do next!!!
As for Ridley, I'm doubting he's even a shoe-in at this point, and I am very worried about his chances. Of course, the lack of shown Metroid stages and Samus' entrance being seemingly cut off in the debut trailer leaves me hopeful, while the Wii Fit Trainer (and to a lesser extent the Villager) leaves me doubtful that Ridley's chances are so high because I fear Sakurai would rather include random or fairly unpopular characters over Ridley.
And on Dixie Kong, being playable in Tropic Freeze might have helped, but her becoming a playable character in the next Donkey Kong Country was inevitable. Only a fool could see it not being a lock.
Also, I'd like to say that DKC: Tropic Freeze could have an impact on smash, as it comes out in 5 months, which is about a year before Smash Wii U and/or 3DS will likely come out. The Super Mario Galaxy and Metroid Prime 3 comparison is a bad one, because Galaxy and Prime 3 came out well after Brawl. However, again, it doesn't negatively effect K. Rool's chances, since he's still clearly the biggest Nintendo-owned character not in Smash (Donkey Kong's 50 million or so sales > any other Nintendo series with less than 3 playable characters with a viable character left), as well as the most popular, sought after, and liked potential newcomer choice. And back to DKC: Tropic Freeze, it can still have an impact without having a stage, like so many games did with Brawl.