But then, who other than the mains of characters that happen to be on the low end of viability could be more vocal about dissatisfaction with the state of balance?
There's a few ways to interpret this question, so I'm gonna shotgun answer a few different ones and apologize if all of them miss.
If you play a character like Greninja, you are totally allowed to complain about things in the game. Even though you play a character that is probably in the top third of the roster, if you just don't have fun fighting Snake then that's how it is--no one can say "No no, you're wrong, you DO have fun fighting Snake!"
But it becomes a little disingenuous to frame your issue as a "balance problem" in any traditional definition. Like, really, that's gonna be your position? That
Greninja should be buffed??? Or that
Snake is the guy we need to nerf? The real problem with this attitude is that once you decree that a performance gap that small is an actionable issue, you've painted yourself into a corner of seeing
all competitive games as unbalanced garbage and advocating for
extremely aggressive/reckless patch schedules. (Which are often not realistic or have a bad track record)
Balance is iterative and things can only become ever closer to perfection, never reach it. There will also be a first and last, always gaps between characters. And people will always complain about the gaps, it's just a question of at what point does it become First World Problems and sort of tone deaf/whiny. You can think the gaps are bigger than other people, or more worthy of more urgent developer action than other people, but those positions extressed strongly can appear weak when they are at odds with historical context. (I.e. complaining about 3% of players playing Ness just sounds stupid to anyone who has ever played another Smash game, or most fighting or MOBA games.)
The issue is not people wanting balance to be better, because everyone should want that. The issue is people talking like the Ultimate flavor of the month is some sort of balance crisis on par with Melee Fox or Brawl MK, that Tekken 7 King is as broken as release Leeroy, or that this week's LoL top pick is basically Day 1 Xin Zhao. When the coversation becomes detached from reality in this way, it puts developers in a lose-lose situation: Either we overreact in line with the magnitude of emotion, or we stick to reality and fail to satisfy what is being demanded.
The best compromise devs can offer is fulfilling a given character's design as well as can be accomplished in any given setting. Focus too much on raw performance and things will get homogenized. Could anyone here come up with a relatively balanced super heavy that isn't some variation on Bowser?
Homogonization is a huge risk; imo it's a chronic trait of amatuer complaints and design-by-committee "solutions." It's correct to worry about it.
But just like polarization, I don't think it's inevitable. (Just the sloppiest, laziest way of accomplishing the goal.)
When I worked on BBrawl I used the term "hat tricks" to refer to changes that reduced imbalance, polarization, and homogonization at the same time. These were rare and somewhat hard to find, but totally existed. They were most commonly found on "obscure tools relevant in bad matchups." In the David Sirlin parlance, you might call them "undertuned Yomi layer 2/3 options."
As a random example, take Pound. Pound is a very signature Jigglypuff move in terms of her aerial playstyle. But it's not her Plan A; it's too risky, and outside of low percents might not be as rewarding. Jigglypuff would much rather (safely!) hit you with a dozen normal aerials and just carry you offscreen. But in some matchups that Plan A doesn't work well because she is zoned out and unsurprisingly those are her worst matchups. Pound, an aerial micro-zonebreaker of sorts, offers a risky way of bursting past swords or whatever. If Pound is undertuned (can be buffed without breaking things), then buffing Pound might not just make Jigglypuff better and specifically improve her worst matchups but also make her
more Jigglypuff rather than less. (This is better than say, homogonizing her by just giving her more range or her worst matchups less range.)
As for super heavies, we can start with DDD and K Rool. Both are doing... pretty respectably! Certainly above any previous expectations of past super heavies. Both of them have vaguely comparable peak results as Bowser (in the ballpark), even though Bowser has much higher usage and broader results at lower levels.
DDD, K Rool, and Bowser don't play all that much alike. A large part of this just comes from the radically different "core neutral tool" they are each endowed with.
I'm pretty comfortable asserting that DDD would be top 10 if you gave him G&W-tier disadvantage; he has a single crippling weakness that has been discussed at length, and is otherwise... pretty compelling at winning his games of attrition. Bowser would be super straightforward to tune to any desired level of performance. Improving K. Rool is the trickier one; I'd want to understand his matchups better before I start prescribing pills.
I think DK is tougher to get right honestly.