I feel this way right now about Joker, and everything in my mind is screaming for me to go back to Olimar as the ONLY local I've taken Joker to resulted in me washing out in Loser's very early on in a tournament without pools.. so that's really doing poorly.
Something to keep in mind is that tiers primarily apply to the highest level of play, and they assume that you've mastered the character. Some characters are fairly hard to learn and will thus require more from you to really do well at mid-levels of play, Joker seems like he might be one of them. While Olimar is a character that is fairly hard to master, he's also a character that you can learn to do well with at mid-levels of play fairly easily, especially in Ultimate where he has some broken tools. If you want to play Joker, you probably have to put in a lot more effort and time to really do well, and even then you might do worse than if you've just stuck with Olimar.
Patches happen, of course, and Olimar seems like a fairly likely candidate for nerfs eventually, so it's hard to say how safe of a bet he'll be along the line. Generally they don't seem to overnerf top tiers in the same game though, all the top tiers in Smash 4 were still top or high tiers after nerfs, major nerfs only seem to happen between games (such as Bayonetta or Sheik).
I think that several years down the line, without too many sweeping changes via DLC, Olimar will probably be the only solo-mainable character remaining at high levels.
How do you figure? Why won't characters like Lucina, Peach, Fox, Wolf, Pichu, Pikachu, Inkling, Palutena, Snake etc. be solo mainable at high levels?
They were pretty common in S4, too, especially among top players.
Two notes: Solo maining was more common and most of the top players primarily stuck with one character (with some notable exceptions, such as MkLeo, Tweek, Nairo, and ANTi). Also, top level players have more time to learn secondaries, so the time and effort spent is less of an issue compared to most non-top level players who generally have less time to learn Smash. Does that mean solo maining is the only option at lower levels? Well, no, especially if you play a mid or a low tier character you could benefit from having a secondary to deal with bad matchups, especially if said secondary is easy to learn (such as Lucina or Wolf). If you're maining, say, Kirby, you'd probably benefit from a Lucina secondary (though you might be tempted to just drop Kirby and play Lucina instead, since, y'know, Kirby's pretty bad).
But even outside of that you see pro players with secondaries for specific math ups that their main cannot deal with well.
Generally it's less about not being able to deal with the matchup and more about gaining an advantage, top level players tend to practice a lot and have more time to pick up a secondary if they feel like it. Still, a lot of top level players seem to essentially solo main, such as Light and Cosmos.
Although, what is your opinion of someone finding match up that is bad, reading up on it and seeing a lot of good players agree that it is a bad MU as well as reasons why, and then deciding to learn a secondary for said match up because even if you get experience with your main you would still be at a disadvantage?
If you're maining a top tier, then you probably won't have any really bad matchups. If you're maining, say, King Dedede, and your local has some good Mega Man players, then picking up a secondary might be a good idea.
Yeah, like from what I have played as well as talking to some Olimar mains (SO to DelugeTN) Pichu just gets destroyed by Olimar, and even if I know the match up it is not the most optimal match I could be playing when I could get comfortable on another top tier like Lucina who can do a much better job dealing with Pikmin, does not die to Olimar's fast smash attacks as early, and can approach safer using sword to keep pikmin off.
Right now, I'm leaning towards Olimar being -1 for Pichu and maybe Even for Lucina, so switching might help but it might not help
that much. Still, Lucina is easy to learn and Olimar doesn't really have many (none, if you trust Myran on this) bad matchups, so Lucina might be a decent counter-pick. Lucina might be a pretty good secondary for Pichu anyway, she's easy to learn and play and should cover most of his bad MUs (especially if you believe that she beats Snake).