I do believe that since the Wii U has had no Mario sports games, or really any Mario spin-off except for Mario Party 10 and Mario Kart 8, we haven't had the chance to really get to see any other character. I definitely think that a huge chunk of Sunshine-related content was prevalent during the Gamecube era since they probably thought that Sunshine was going to be a big hit, or it was just easier to bring over characters from it. I mean, it's like Luigi's Mansion and Sunshine they really drove all the way through the Gamecube era.
But I don't think they are forgotten or gone. I am baffled about what happened with the many missing characters in MK8, but Petey Piranha is still constantly used, with his latest console appearance in MP10, and Toadsworth appeared just two years ago, so it's not like he's been missing since 2008 like some characters were before MK8 came out (cough Baby Luigi cough).
And Birdo has been still used a lot. She was in Sticker Star, and she was a playable character last year in Mario Golf: World Tour. I don't know what is up with her and MK8/MP10 (though the latter may be a good thing), but I sincerely doubt they forgot about her. Even in MK8, they had banners with her logo and stuff on it.
As for the Ultra Smash game, we only know of 5 playable characters at this point. It's too soon to say who will be appearing/making a cameo and who will not be.