Yoshi's pretty much separate in icon only as far as I'm concerned. It'd be like if Luigi was listed as a "Luigi's Mansion" character. Only separate on technicality.
It's not like Donkey Kong, who has very little ties to Mario other than both DKC and Mario Bros. branching off from the same source of origin, the Mario vs. DK subseries, and DK characters joining in for sports, kart racing, parties, etc., or Wario, who's practically got his own universe in WarioWare and has had no ties to Mario in Wario Land since the first game, only showing up for sports, parties, kart racing, etc.