Disagree.
Every character is viable.
Can you explain your point further ?
Viability : capable of working successfully; feasible. of course we're not talking about locals when discussing viability, The majors / super majors are what we're truly thinking about, so who's capable of working sucessfuly ? ( capable doesn't mean that it already did win, only that it showed demonstrable potential to ).
For example, Bowser has never shown any potiental to win a major, no top 8 reached, no notable wins on players from the top 100, while Yoshi, a character that never did win, showed that he could, with Amsa having wins over pretty much everyone on top 10 and having already reached top 8 loads of times. Only thing holding Amsa back is the coronavirus.
Also, when Amsa plays, he doesn't add anything to Yoshi, its still the same character, if anyone trained for a long enough time and with a good dedication, they could get the same results that Amsa does, what gives him the wins that he has is that he uses the tools that Yoshi has incredibly well.