I'm not quite sure you can say that Pac-Man, one of the biggest icons of video games to ever exist, is not on the same level as Mario, Mega Man, and Sonic, and then say that Layton, the protagonist of a small but popular DS series, is.
And this is coming from a guy with a Professor Layton based avatar.
Pac-man was a revolutionary game, but only the original game actually meant anything (the only other Pac-man games with wide critical acclaim are remakes like Ms. Pac-Man and Championship Edition). And most importantly - it had NOTHING to do with the character.
And while this applies to Mario too, he later developed a recognisable image beyond his original sprite. Pac-Man didn't. The Pac-Man everyone recognises is the original sprite. And not because he was jolly like Mario, cool, rebellious and expressive like Sonic, or relatively stoic and bad*** like Mega Man.
Pac-Man is not iconic based on his merits as a character, he is iconic because there were hardly any other games around at the time. It's much less the character that is iconic than it is the game. I know literally one person (on the INTERNET no less) who claims to care about Pac-Man
as a character. Anyone else just plays Pac-Man for five minutes on their phone while they are taking a dump. If not that, they play it competitively. But they don't give a crap about the character and aesthetics.
And before it's argued "well Pac-Man could represent the dawn of video games", Mr. Game & Watch says hi.
I'd also like to point out that Ghostly Adventures is Namco's desperate attempt to make Pac-Man relevant as a character and as a franchise. If he was really such a relevant character, why would he need this treatment (and why would it have failed)?
If we really needed a third party mascot to fit in with Mario, Sonic, and Mega Man, to complete some kind of circle, it'd be Crash Bandicoot. He's the only third party platforming mascot that I feel would DESERVE the place in Smash. He's bascially like Sonic, in the sense that he used to be competition for Mario.