Regarding the "no sauce" newcomers:
-I would not really count Waddle Dee and Impa under this umbrella at all--both would have something head-turning by default with the simple appeal of being the first characters added to their respective series since Brawl (which will likely be 18-19 years ago by the time we get the next Smash), the former also likely heralding more recent Kirby content being added to the game, and the latter having multiple possible approaches with how she could fight.
-The longer Smash goes without Dixie Kong, the more and more interest is lost from people, it seems. I think that's somewhat reasonable with the scope Smash has been building up to over the last 20-ish years, and it doesn't help that in terms of uniqueness, at best I only see her being a "Falco" on a scale from "Daisy" to "Lucas" when it comes to how distinct from Diddy I envision the devs making her. I think backlash could be easily minimized if she's not the sole reveal in a Direct like Isabelle was last time, though I still think the general consensus will range from "Oh, nice" to "About freaking time, jeez". K. Rool undeniably stole a lot of thunder by arriving solo in Ultimate.
-Others have explained the problem of Kamek being a league behind other Mario and Mario-associated picks, but for me, I think the biggest nail in the coffin is that he is indistinguishable from generic Magikoopas, and the Mario creatives seem to have no intent to change that beyond Nintendo Cube occasionally giving him a bow tie. It feels like Nintendo has no real faith in him as a unique character beyond just making him the only Magikoopa that appears in some games. Kamek just does not feel fully realized compared to...say, Toadette.
-Which brings me to Toad, who I honestly think fits the description best, as much as some dislike to hear it. Personally, I have my own trifecta of "dull picks"; Toad, Oatchi, and Tom Nook, but I'll get to that topic in a moment. To put it simply for now, Toad's probably the one "serious" Mario pick who I'd say still has a notable negative reputation in certain Mario circles. Sure, some people got upset at Waluigi fans freaking out after Ultimate, and Geno has dedicated ****post trolls surrounding him, but Toad's probably the one recurring character I still see shade being thrown at after hatedoms for characters like Daisy, Rosalina and Waluigi fizzled out. And I honestly can't blame them with how plain Toad can feel as a character sometimes--it says a lot when I see a lot more people being fans of Toadette specifically over Toad. And I won't deny that I'm in the same boat--I feel like I'd enjoy Toadette by herself as an inclusion way more than if she was relegated to a Toad alt. Toadette just has more of a personality to her, and they've even given her quite a few different things to do over the past decade.
But anyway, as I've said, Toad, Oatchi and Nook are my personal "no sauce, dull inclusion" characters. The former two are pretty easy for me to explain--Toad to me represents Nintendo's phase of trying to "dial back" the Mario brand. It's an overplayed statement, but it genuinely is hard to disassociate bog-standard Toad from the white-breadness of the NSMBWii aesthetic that took over Mario for several years, or the paper-thin variety of NPCs in most of the 2010s Mario RPGs, or the Switch Mario Sports titles that tried to skate by with not much content. He just kind of manifested as a "face" for the Mario series's later missteps in my eyes.
Oatchi just honestly feels like a design that is engineered to be cute in a not-entirely-honest way--think Grogu from the Mandalorian and similar designs like that, and it just kind of turns me away on principle as a result, and that's not even going into the character being blamed for Pikmin 4 being "easier" compared to previous installments (I have no say if that's true or not, I've never played Pikmin). Something about the character just feels forced to me, and I'd personally be content with someone as an Olimar echo instead--or giving him more characters as alts besides just Alph.
I've argued about Tom Nook in the past so I'm just going to throw all pretense out of the window here--something about his design irks me in an unexplainable way and I'd groan at the idea of Animal Crossing going the way of Star Fox or Fire Emblem with how little the series varies in body types--except at least the other two franchises can have characters that wildly deviate from the standard body type, AC quite literally only has slight variants on the chibi bodytype. The proposition just honestly feels snore-inducing to me personally. It's kind of funny how I'd likely be more intrigued in how someone like Officer Howard or one of the XB3 leads would play at this point--they may be "standard anime-esque humans", but I'd honestly take that over how...dull and edgeless the other three I mentioned are in my eyes.