Professor Layton would be a great addition to Smash Bros.! Though I haven't really played any of his games (I own Professor Layton and the Miracle Mask but I've barely played it), I've always thought he should be in Smash post-Brawl. I didn't think there was much of a realistic chance of that happening, though, since he's kind of in that weird gray area between third-party and first-/second-party. I mean, yes, technically he's third-party, but he's essentially a Nintendo character regardless of that, having only appeared on Nintendo systems with his games being published by Nintendo outside Japan. It's kind of comparable to Mega Man on the NES, but even more closely related to Nintendo.
If you compare him to the other third-party characters in Smash, no, Professor Layton isn't quite on the same level in terms of being iconic. To be fair, though, Professor Layton is a much newer character and franchise than PAC-MAN, Ryu, Mega Man, or Sonic, and you could argue that Professor Layton is a modern gaming icon. But regardless, maybe we shouldn't be comparing Professor Layton to other third-party characters, but rather to other Nintendo characters, since he essentially is one in many ways. In that regard, I think he definitely fits in and deserves a place in Smash.
Plus, Professor Layton would finally bring some good Nintendo DS representation to the roster. For Nintendo's most successful system, the DS sure is underrepresented in Smash Bros., at least when it comes to fighters. Currently, Lucario is the only character born on the DS in Smash Bros., and it's not even from a franchise that originated on Nintendo DS. While I've been (and still am) hoping for Chorus Kids as a DS rep, Professor Layton fills that role even better, coming from a franchise that debuted on the DS. Actually, I can't think of a better character to represent the Nintendo DS than Professor Layton.