The question is always: what low tier, viable relative to what benchmarks, and for what goal?
Going by official 4BR tier labels...
S:
S rank characters are expected to win nationals, but this is largely a function of the players standing behind them. IMO, we'd be equally surprised if someone beats Zero/Nairo/Dabuz regardless of it they played an S or A tier character.
A:
B:
S, A, and B characters do not surprise anyone when they win a major tournament. We are also not overtly surprised when any given regional event does not have a single player of a particular S/A/B tier character. Even Sheik not making top 8 at most regionals would barely register beyond an eyebrow raise. B tier characters are merely regarded as a little less likely to be able to run through a bracket.
C:
C tier characters could be regarded as equally formidable, but have never proven themselves on the
national level. Below that, they are
incredibly well-performing: Falcon has the 2nd highest aggregate results on Smashboards rankings, and Yoshi is sitting at #8. (Both are above ZSS, Rosalina, Ryu, Pikachu, and Sonic!)
D:
D tier characters would be be mildly surprising
but not at all shocking to see in top 8 of a national, and they win regional events all the time. Lucario, the lowest D-tier character, won two massive tourneys in Japan and Mexico just this weekend, taking down Rain
and Leo.
E:
E tier characters have also seen dominant regional performances, just in more isolation and at the hands of fewer players. There are many areas on this planet where a Greninja or DK winning an area tourney is not just unsurprising, but the status quo.
F:
F tier is where we start being surprised (but still not shocked!) to see win a regional event. However, these characters still have a history of performance at the hands of specific individuals, be it Tweek's Bowser Jr, Regi's G&W, or Wave's WFT. We are not remotely surprised to see those individuals in top 8 of a regional.
Also note that almost all of these characters received
significant buffs over the last two patches, so they can honestly be regarded as candidates for true mid-tier positions.
G:
G tier is where we start getting a little shocked to see win a regional, and generally surprised to make top 8. However, a few special players we
aren't surprised to see place with them: No one is surprised when Brood performs decently with Duck Hunt or when Sol does well with Little Mac, and no one will forget Nairo's use of Dr. Mario.
H:
I:
J:
H/I/J tier characters would surprise people to break into top 8 at a regional event, but can still perform very well at locals. I've personally won multiple competitive events with Ganondorf and Jigglypuff in Smash 4--and if I can do it, others sure as hell can.
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Let's compare the top and the bottom in terms of Smashboards aggregate rankings, excluding DLC and Miis as outliers:
2936
1506
1522
1419
1487
454
393
571
466
247
In other words, the worst characters place around
24% as often in tournaments as the best characters. This is
without accounting for exceptional players that make up a disproportionate percentage of results, or that better characters tend to be significantly more popular in tournament usage to begin with.
They are very much on the radar; only Zelda comes close to slipping off.
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In Brawl, I won several events in my subregion and earned a little over $1,000 over 3 years playing Brawl Jigglypuff, who is a worse character than anyone in Smash 4. That's not much on the national scale, but I would speculate that this meets or exceeds the benchmark that most people would (or should) be considering, and shows that character tier is not the limiting reagent towards that goal.
If you genuinely aim to
win a national, you would be wise to focus on a S/A/B tier character.
If your aim to
win an entire regional, a C/D (and maybe E) tier character is also a completely viable choice, if appropriately handicapped.
If your aim is to place in the top 8-16 at regional events with some consistency (and top 64-128 nationally), E and F tier characters will also be sufficient. At this level, you will probably find that the advantage in matchup unfamiliarity to largely counter-balance the tier difference.
If you just want to win or place well at local tourneys, then any character is fine. Just be aware of the modest additional difficulty that G/H/I/J tier characters may bring to the table.
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In teams, note that almost all of this goes out the window, because the tier list looks significantly different and combination matchups have a big influence. The bottom 3 characters on the list (Ganondorf, Jigglypuff, and Zelda) are all disproportionately better in teams than singles.
My bottom line advice: Set realistic, near-term goals, and use whatever character you perform best with today to get there. It is exceedingly rare to rise to a level where your character is
legitimately the main thing holding you back, and well, if you are in the situation, you are thriving as a player.
Burn that bridge when you get to it, but today, focus on today. Smash 4's balance is good enough to allow that without much hesitation.