I don't think that being the first in whatever is a strong point for a character to get in Smash. It's gotta be more than that... look at Ninten and Ness. Ninten is the first protagonist of the Mother series and yet Ness is the one who got in. You listed Mr. Game & Watch and R.O.B. but they contributed a huge part in Nintendo's history and sucess and it had nothing with being the first something.
And since you (not me) brought up Duck Hunt Dog. (he's sorta my Impa
) Duck Hunt helped contribute to Nintendo's history and success during the NES era, thanks to the NES Zapper. Now, yes, Wild Gunman is the first NES Zapper game but it didn't popularize the NES Zapper like Duck Hunt did. There's a reason Duck Hunt and not Wild Gunman was packaged with the NES in the West. The NES Zapper was succeed by the SNES Super Scope and later succeed by the Wii Zapper. The gaming concept of pointing your controller at the screen originated in the arcades and the NES Zapper brought that into our homes. I guess one could argue that Duck Hunt Dog could be used to represent the NES Zapper (hey, a Nintendo hardware!) and the light-gun genre (a genre that originated in the arcades as well).
In other words, being the first is not good enough unless we are going to add Pauline in Smash for being the first damsel-in-distress even though Peach popularized the video game archetypal role.