I think that, for most cases, that you don't strictly
need a secondary or co-main unless you're playing at the level where you're getting far at majors/supermajors and getting actual significant pot bonuses from your play. Because tier lists and MU ratios are based on players of equal skill, and skill fluctuations are far more significant at the local, regional, and lower levels; tier lists and MU ratios become less relevant as the level of play decreases. Not that they aren't relevant at local and regional levels all - they're just less so than at the absolute top level of play, where fluctuation in skill between players is much smaller.
Armada discusses a similar topic in one of his videos:
And that's in
Melee, which is quite a bit less balanced than Sm4sh.
That being said, it certainly doesn't hurt at any level to have a secondary or co-main if your main has some egregiously bad MUs. Or if you just don't like a MU with your main. As an example, there's one (and only one) MU where I'll switch to a secondary character, and that's more because I find the MU unfun to play with my main rather than it being actually bad (though it is indeed a horrible MU).
Long story short, it's not necessary to have a secondary/dual main at most levels of play, but it can still be helpful.