I do agree that it does feel like there is too much Mario series representation, both in terms of the amount of characters they have, the amount of items they have, and the overall amount of representation they have in contrast to many other series.
There are many Nintendo series unrepresented through playable characters, many series which only have 1 rep, or have the same character a 2nd time, just in a different form. It's not like these other series don't offer potential for unique and/or popular characters, and it's not like the Mario series needed to have as many reps as it does.
Dr. Mario was hardly needed as an inclusion, and I do not feel Rosalina & Luma have merited their inclusion as of this moment, with the only truly strong cause to get them included being her recent playable role in Super Mario 3D World. Bowser Jr always struck me as the next most deserving Mario series inclusion, but in the manner they have included him, with all of his palette swaps being the Koopa Kids, it comes across more so as playing as the Clown Copter rather than as Bowser Jr. Furthermore their particular styles of move-set, be it Rosalina's puppeteer style or Bowser Jr's vehicle style, could have been utilised by other characters from other series.
One of the arguments I hear for the Mario series having so many reps is so that it would not have less reps than Zelda or Pokémon, but there rep amount seems bizarre as well. The Zelda series only have 5 reps due to the removal of transformations and the fact that they have two versions of Link in the form of Toon Link returning. Ignoring my personal dislike against the removal of transformations, the return of Toon Link doesn't make much sense given his lack of change, his return would be more approvable if more had been done to differ his special moves (as in, giving him Wind Waker, Phantom Hourglass, and Spirit Tracks representing weapons). Then there is the Pokémon series, to whom I do not understand why Charizard returned. With the removal of transformations, it was an easy case for Sheik and Zero Suit Samus to return as a single rep addition to their series, but in the case of Pokémon Trainer, their shouldn't have been a choice between having just one of them. That could just be my feelings on the Pokémon Trainer's inclusion, how I feel it was merited by representing a unique aspect of the Pokémon series and how the Pokémon used by the trainer individually didn't merit inclusion due to their representing factor being just another Gen 1 rep. Neither Zelda nor Pokémon had inclusions that necessarily required above all else to have 5 reps, eliminating much of the need for Mario to have 5 or more reps (though out of the case for having 5 reps, Bowser Jr inclusion seems the more deserving).
The Mario series is the only series within SSB to have what is effectively spin-off series represented as playable characters, such as DK, Yoshi, and Wario, who all debuted from the Mario line of games (well, one could argue that they all originate from DK instead, but the point is they are connected). It is of my opinion that having further representation connected to Mario would have been best done through additional reps in the DK, Wario, and even Yoshi series. The DK series does have Dixie Kong and King K. Rool, both popular, well known, and deserving candidates for inclusion with plenty of unique potential. Ashley is a very popular character and there was quite an outcry when she was confirmed as an Assist Trophy, a playable role could have worked well for her as a Wario series rep. Perhaps Wario series representation could have even gone in the direction of Wario Land representation and included Captain Syrup. As for the Yoshi series, I've always found it somewhat too connected to the Mario series (given Mario and/or Bowser still features in the Yoshi games), but this works in it's favour here in this argument in how Kamek would have done well as both a 2nd Yoshi series rep whilst also expanding the Mario series influence without adding to the Mario series representation.
Of course, in the end it is all a matter of opinion. For myself, I am displeased the Mario series has 7 reps whilst Kirby has only 3, DK has only 2, Metroid has only 2 (whom are both the same person, just in different forms), F-Zero has only 1, Wario has only 1, and Yoshi has only 1. I'm disappointed that we have 7 Mario reps whilst the Mother series was reduced to having just 1 rep, and the Pikmin series alternative skin wasn't made into a separate character unlike the Mario series alternative skin case. I can understand why Animal Crossing and Punch-Out don't have more than 1 rep, given how they've only just gotten playable representation, but I'm still disappointed that they've only just joined at a time the Mario series has 7 reps. I'm disappointed that the Starfy series, the Golden Sun series, the Panel de Pon series, the Rhythm Heaven series, the Clu Clu Land series, the Custom Robo series, or the Advance Wars series have not even got a single playable representative at a time wherein the Mario series has 7 reps. That after all though, is just my thoughts on the matter.
