In short, I think it's a plausible argument to say literally any character is viable with time, patience and skill, but the actual chances are just a whole different thing altogether.
Sure, I don't think we'll see Zelda (for example) pulling anything flashy off in a tournament all by her own any time soon, but if someone comes along who can really make the most of low-tier tools, we might see some spectacular play (-and actually already kinda have.) from said characters. Whether that classes them as viable or not, I dunno, but I think some people just have a natural affinity for playing these lesser represented characters, and it really does a lot for the community when they stand out.
I'd also argue that low-tiers have some benefit in the sense that a lot of people aren't specifically prepared for their strengths to come through, so they can catch a lot of people off-guard. I mean, Nairo's Doc certainly caused a storm a while back, and sure Nairo plays ZSS mainly, but I'd dare say that his Doc play could carry him a long, long way if he took to it. It's not completely impossible.
-and I'm not going to judge players like Nairo for taking the "easy route" with ZSS because actually, a lot of higher tier characters take more skill to pull off well and consistently (-and then you have the argument that they're often playing for money and titles, etc. so it matters more and risks are a lot more harmful.), so judging them for that is kinda stupid, but I'd personally love to see people pulling out and sticking to their lower tiers just to prove a point that any character can make it. However, tournament scenes kind of breed a type of player who doesn't like to take risks all the time, and that's fair to them, really. For some of them, it's basically their livelihood.
So in a logical sense, I believe that, yes, all characters are potentially viable. In a meta sense? -no, they're not. -and that's just down to the players and the mindset of the game, I feel. It's not a bad thing, by any means, since these people often have a career to keep going and play exceptionally entertainingly regardless (Infact, the idea of people playing at their peak is often more hype than an underdog low-tier rising up.), but when I see people pulling out the low-tiers, I get a sense of excitement at seeing something new being tried and practiced out in a competitive environment, and to be honest, I think the lack of that really puts me off watching a lot of it...
Still, I feel I digress. Regardless of personal opinions on viability, one thing people can agree on is: Smash Gods bless our low-tier heroes.