If you ask me ROB only got in because he played a major part for Nintendo in the 1980s, the few reasons why Nintendo is even here today, he really shows how he pushed Nintendo in through his playstyle, destroying every character with one move just like Nintendo vs it's competitors in the 80s.
Mr. G&W is a bit different, he might have gotten in just because of his age, or maybe he got in because he really doesn't have anything left to do (G&W gets more love than F-zero, which is a bit sad) G&W may represent when gaming wasn't a big deal until he showed up, coming in with crazy weapons no other character uses.
Duck Hunt, they were one of the first Nintendo games, and first games in general, to show how diverse Nintendo really can be, and even Wii fit trainer got in because of this reason.
You see, every weird character got in for a good reason.