I would list Pikachu Libra, Leaf, female Robin and female Corrin too but I'm not interested to get into arguments with certain people over this. (I do considered them as their own characters as well. Even Likachu Libra, since she's the representative of Pokken despite her moveset isn't accurate tho.) I'm sure that there are more.
I don't really count those mii costumes since they doesn't really looks like real characters but Sans really comes so close tho.
I would argue Sans is the only Mii Costume currently who could be considered an entirely accurate representation of his character (He's entirely "on model" to use an animation term). His proportions and features match almost exactly the official figurines that have been released for him through fangamer.
With that in mind, I would count his costume, a full body replacement for Mii Gunner, as a legitimate appearance of the character in the game.
What if we were to take the commonly accepted number of "unique" playable characters (ones with individually displayable slots on the character select screen) in the game currently (81 including Terry) and add in all the canonically separate characters who are alt costumes?
Sans
Dark Link (?)
Pikachu Libre
Blood Falcon (?)
Galacta Knight (?)
Dark Meta Knight (?)
Leaf
Claus (?)
Alph
Male Wii Fit
Female Robin
Larry
Roy
Wendy
Iggy
Morton
Lemmy
Ludwig
Female Corrin
Inkling Boy (??)
Meta Ridley (???)
Erdrick
Four
Eight
*"(?)" denotes a costume with elements of a separate canonical character that are not entirely "on model" given their appearances in past games
*"(??)" While the only canonical game universe version of Inkling selectable is Agent 3, The Splatoon manga has a bunch of Inklings who look very similar and are all distinct characters. Honestly I kind of just threw my hands in the air on this one.
*"(???)" There is legitimate debate within the Metroid fanbase as to how exactly Meta Ridley's appearances work canonically when Ridley is seen literally blowing up on several occasions. Some theorize that Meta Ridley is an enhanced clone of the original Ridley, but it is unclear if it shares the consciousness of the original or is a distinct character
Adding the above 24 to our the number of 81, our current total number of playable canonically distinguishable characters in Smash Bros Ultimate is at
105 CHARACTERS
Did I miss anyone?