Donkey Kong, Metroid, and Kirby are all underrepresented from my point of view, but unfortunately, without Ridley and Dark Samus, Metroid is in a weird state where it has no suitable fighters to fill that gap. I find Donkey Kong's under-representation to be simply unacceptable though, it's ridiculous a franchise the size of Zelda gets so little love. Lastly, Kirby is one I don't see mentioned a lot in the West, while in Japan, this franchise nearly gets as much outcry as DK for being underrepped and it's for good reason, it has a quite the problem in that literally no stage or character are from the Post-Sakurai era, and that it's the only first party franchise that isn't retro to not receive a stage at least as recent as 2004. Not to mention it's Nintendo's 5/6th largest franchise (Depends on if you count Wii as a series) and has less representatives than a franchise that has sold less than 17 million, which smacks of bias in my eyes.
For over represented, I'd go ahead and say only Kid Icarus and Fire Emblem really have this problem. Both of these franchises are beloved by Mr. Sakurai so it's understandable that he'd want them to receive as much love as possible, but I can't really accept the current state of their representation, where Fire Emblem, who I'll remind us has less than 17 million sales has the exact same representation as Zelda, ZELDA. Two clones? Check! Three unique characters? Check!
I have nothing against Fire Emblem or Kid Icarus as franchises, and while I understand their fanbases want them to be more important in the future, I do have a quite a bit against them being raised on some magical pedestal that lets them inflate their worth, and it feels even more insulting when it would make it appear as if Fire Emblem is above Kirby and equal to Zelda, which is simply untrue.
Also, one question, what makes Fire Emblem special and different from the others? The reason I'm asking this is that this is a justification for Fire Emblem reps that I get a lot, but don't really understand. I at first thought they mean't it's quality, which is purely subjective and is kind of a free pass, but apparently that wasn't what it is. I then thought it mean't how much history it had but then I realized FE only has 15 games, while Kirby has 26, so I realized that couldn't be what they were mentioning and now I'm just really confused? If anyone knows what that justification/statement means, please tell me, it's the one thing I've always wanted to understand but can't since I don't know what it means.
Edit: Grammar, added the word "thought" to where it was missing.