Meowth is by no means impossible. Yes, he is unlikely, but he's not impossible or even worthy of a 1%.
I would still categorize Meowth as a Nintendo All-Star, and one of the very few left to be playable. I'd say he's on the same level of being an All-Star as Ridley, Toad, Dixie Kong, or King K. Rool (though I'm not implying he's nearly as likely as any of them). The reason I think Meowth is one of the best Pokemon choices is because Smash isn't about relevancy. Yes, it is good to have an plug or two in Smash (Ike, Lucario, etc.), but I don't think it should define the whole roster. Characters like Chrom or Shulk have been considered some of the most likely choices as of late, but haven't even been established as Nintendo All-Stars yet. Meowth was established and became an All-Star all the way back when the anime began. Even if he's not as popular as he was back then, he still bears some popularity and is still relevant in the anime.
The game should not be the only thing that influences character selection for the Pokemon series. Yes, it is the biggest part of the franchise, but it isn't everything. You say that Ampharos and Marill make more sense than Meowth just because they have a bit of relevancy in the 6th gen. But does that really matter as much? In a few months, sixth gen will just be one gen in an ocean of them. That's like saying Dunsparce would be a better choice than Meowth if he recieved a Megalution. Meowth is still much more well known and iconic than Ampharos, Marill, and most other Pokemon. Nothing revealed from 6th gen has been related to Meowth. That is true. But there is still plenty of things to be revealed for the sixth gen. We're probably going to see dozens upon dozens of new features and evolutions to old pokemon, so you can't totally count Meowth out yet. GameFreak probably doesn't care about 95% of the pokemon. There is going to be over 700 Pokemon by the time gen 6 comes out, so it's going to be incredibly difficult, boarder line impossible, for GameFreak to show care for every single Pokemon. Among Pokemon reps, I don't see many that outdo Meowth. The only incredibly obvious one is Mewtwo, and then we're left with Blaziken and a bunch of fifth gen pokemon.
A common trend in the Pokemon series in Smash is for the two new reps to be a plug and a classic, iconic character. In Melee, Pichu was there to represent the newest game, and Mewtwo was there for being one of the most popular Pokemon and being very iconic. In Brawl, Lucario was there to represent fourth gen, and Pokemon Trainer was there consisting of the classic starters and representing who you actually play as. In Sm4sh, why not add Mewtwo to represent his relevancy in 6th gen, and Meowth as our iconic character. I also think Meowth has a lot of potential to be unique. Much more than a lot of other Pokemon. I understand your reasons for disliking Meowth, and they do make sense for the most part. I still find him to be unlikely. But I don't think Meowth is worth of a measly 1%.