If I had to interpret everything Sakurai has said about "retro" characters:
: Actually were chosen to be a throwback character from the NES era for people who grew up with NES games.
: Surprise character, probably added for the historical significance of the Game and Watch to Nintendo's history (first product line to sell more than 10 million copies. First runaway commercial success for the company.)
: Was added because he wanted to do a modern reimagining of the character, in a similar vein to how Link has changed over time. He probably was already flirting with a Kid Icarus revival at this point.
: Surprise character. Probably added since it arguable saved the North American video game industry.
: Could be argued as a retro from a Japanese perspective, but had recently gotten a modestly successful Wii revival and was a popular request in the West. Sakurai focused more on the arcade "retro" elements of Mac over the NES version too, which goes against the supposed "retro" theory.
: Surprise character. Massively iconic character from a hugely popular game that came bundled with the NES in North America and Europe. Helps that the light gun and Nintendo go way, way back. (Nintendo developed the light gun for the Magnavox Odyssey in Japan, and had several light gun electromechanical toys/games starting from 1970, including a
1976 Duck Hunt eletromechanical home toy/game. This makes the Duck Hunt series the oldest series represented by a character in Smash Bros., though Wild Gunman as a series was started with a
toy in 1972 and an
arcade game in 1974, making it older.)
: Surprise character. Not really retro, since it references New Super Mario Bros. games in its moveset. Likely just a surprise because of its role not being a major hero or villain, just a regular enemy. Still iconic, of course.
Then there's all the third parties, of course, but Sakurai views them differently and values the legacy of their franchises, often honoring them by making them faithful to their best-known games.