The way I see it, Tingle is just a recurring NPC throughout the Zelda series, and, as far as I know, has never played a role in the main plotline of a game. His own games would probably count as a separate series, but the main thing that cripples his chances is that those games weren't released worldwide and that Tingle is so god****ed hated in three fourths of the world.
Waluigi, however, is a playable character in a multitude of games, some of which are among Nintendo's all-time best selling games on the Gamecube and the Nintendo DS. So he's never been in a main platformer? I honestly doubt Nintendo cares. They probably think of the Mario sports games as legitimate as any other Mario game and I don't think they're going to act all elitist with their own games. If the game is popular enough, party game or not, chances are, we may see rep from it. Just look at WarioWare.
Besides, the Mario series is running out of main platformer reps, (basically only Bowser Jr.'s left) so they may have to expand to the Mario sub-series'. Out of all of them, the most popular Mario series aside from the platformers is undoubtably the Mario Kart series. Judging by sales, the next most popular series is, as much as it pains me to say it, the Mario Party series. The Mario Strikers games seem to be well on their way as well.
For these reasons, I say Waluigi actually has a fairly decent chance of getting in. Personally, I'd prefer him over Tingle anyway. Seeing Tingle fight is.....just.....plain....WIERD!!!