Eh, iconic/memorable can easily debatable. Just because I don't see Dr. Mario as iconic, Sakurai did. To me, he is the same as Mario. In fact, he's a spin-off game of Mario, which is why people argue that Waluigi cannot get in... Again, if Sakurai wants him in, then he'll be in.
Besides, the fans have more input than that. Little Mac, for example. From what I've gathered, most of Japan were shocked at his inclusion. Some didn't even know who he was. Besides, Sonic got in. Not because he is iconic, memorable, or recognizable. Those are three traits that he possessed, yes, but that is not why he got in. The fans wanted to see Mario vs. Sonic. And, thus, he was added.
And the game from which Geno came from was a big step for Nintendo. There are tons of people here who could go into details about it, but just check the front page of the Geno board for Sm4sh or Brawl. The Brawl board was filled with info for him. There is always a chance, and Geno was one of the top characters on the Pre-Brawl poll, correct? (If I am wrong on that, then someone please correct me.)