What is considered as the Yoshi series in Smash is a bit odd, when you have elements that could be classified as part of both the Mario and Yoshi series at once. Instead of arguing about a potential newcomer for the Yoshi series, I'll be doing a short analysis.
Smash 64 had little content, with Yoshi and the Yoshi's Island stage (inspired by Yoshi's Story) as being classified as part of the Yoshi series. Keep in mind that in 1999, aside from spin-offs, Yoshi hadn't been in a major Mario title for a while. Sakurai must have thought that the series would branch out similar to DK though it never did.
Melee adds two Yoshi stages and some trophies. The stages are from Super Mario World and Yoshi's Story. Going by the order of series, the theme song for Yoshi's Island is classified as a Yoshi series song, as it's between Metroid and Kirby. The order follows their debut: Mario, DK, (1980) Zelda, Metroid, (1986) Yoshi (1990), Kirby (1991), Star Fox (1993-1994?) and Pokémon (1995).
The alternate song for Yoshi's Island is considered part of the Mario series, being the first song to appear among the alternate ones (in the sound test's ordering), those songs following the series order I mentioned above.
Yoshi trophies (besides Yoshi's fighter trophies): Shy Guys (using their design from Yoshi's Story), Pak E. Derm, Baby Mario, Baby Bowser and Raphael Raven. Mind the year of Melee's release: 2001. Most of these trophies were characters exclusive to Yoshi games back then, with the sole exception of Shy Guys. Even so, Shy Guys hadn't been in many Mario games back then, sticking primarily to the Yoshi series, plus they were using a design from Yoshi's Story.
Brawl adds a Yoshi stage based on the first Yoshi's Island game and Yoshi's Island DS, taking inspiration from those games' art style. Yoshi's Island from Melee returns and still classified as a Yoshi series stage, even though all other content from Super Mario World, notably music, is classified as part of the Mario universe, bearing the Mushroom symbol.
Yoshi series trophies include Yoshi's fighter trophies, Baby Mario and Baby Peach. Shy Guy is now a Mario series trophy, using the 3D design from modern Mario games and spin-offs. Brawl has notably less Yoshi series trophies than Melee, with the removal of many elements from Yoshi's Story. Stickers still feature some official artwork of Yoshi series exclusive content, such as Yoshi's transformations, the Stork and enemies like Raphael Raven or Goonies. In addition, the sticker depicting official art of Yoshi from Mario Party 2 is included as part of the Yoshi series.
The 3DS and Wii U games barely feature any Yoshi series trophies. Baby Mario and Baby Peach now appear as Mario series trophies. The only non-fighter trophies are 3DS exclusive, those being the Fly Guy (regular Shy Guys are Mario trophies) and Yoshi's New Island (featuring Yoshi and Baby Mario as they appear in that game).
On the Wii U version of the game, the Woolly World stage is added as part of its stage line-up. The two Yoshi stages featured in Brawl return, split across versions: Yoshi's Island from Brawl is on the 3DS version and Yoshi's Island from Melee is on the Wii U version.
Here comes the interesting part: the Yoshi's Island stage theme from Melee is now classified as part of the Mario series, despite being the main song from a stage classified as part of the Yoshi series. All other Super Mario World content is considered part of the Mario series. This means that Melee Yoshi's Island's music selection is all considered as part of the Mario series, with the exception of the Yoshi's Story song remix from the first Smash Bros. game. The rest of the Yoshi series songs are in Woolly World, including songs from Brawl and the Melee remix of the Yoshi's Story theme. Still, the stage features some Mario songs such as the Paper Mario Medley.
Smash 3DS/Wii U also has Yoshi placed between Bowser and Rosalina, two Mario series characters, in the character selection screen.
It seems to me that the Yoshi symbol is now used to denote content strictly exclusive to the Yoshi series and Yoshi himself, the sole exception being the Super Mario World stage (which itself is named after Yoshi and went to the main setting of the Yoshi games, so that might be the reason). Many elements first classified as part of the Yoshi series are classified as part of the Mario universe in the latest Smash Bros. game, which could be reflecting those elements appearing more often in main Mario series, unlike during Melee's time of release where they were more prominent in the Yoshi games.
The DK and Wario series still have a lot of distinct content, certainly more so than Yoshi. Even so, the trophies for DK and Wario in their karts appear as part of Mario series. Similarly, stickers in Brawl pertaining to DK and Wario in Mario sports games are also classified as part of the Mario series, acknowledging in a way that those series are certainly part of the same greater universe.
Basically, the DK, Yoshi and Wario series tend to be used in Smash to denote content exclusive to their own series, in addition to the characters themselves. Likewise, the same character can be part of two different series depending on the context of the content: Shy Guy's trophy in Melee depicts their appearance in Yoshi's Story, so it's part of the Yoshi series. Likewise, Donkey Kong and Wario's trophies as they appear in Mario Kart (driving their karts) are counted as part of the Mario universe.
Still, there are some exceptions, such as Yoshi's Mario Party 2 and Wario's Super Mario 64 DS stickers in Brawl being classified as part of the Yoshi and Wario series. Sheesh, even Smash Bros. has trouble classifying the different series.
Goes to show how thin the line separating these four series are, given they share so many common elements and spun-off from the same series.
