I’ve seen people have some misconceptions what it means to be a low tier/high tier and such. First off, just because a character in low tier doesn’t mean they automatically lose to every high tier, that’s wrong: Low tiers are low tiers because their overall matchup spread is bad, but that doesn’t mean any specific matchup is necessarily bad. Jigglypuff in Smash 4 (one of the worst in the game, worse than Ultimate Little Mac probably) arguably went even or beat Captain Falcon, a high tier or high-mid tier character. A top tier character can go even or potentially even lose to a high-mid character, that doesn’t mean the high-mid tier character is suddenly top tier as well.
So, with that out of the way, we should try to understand what makes a tier list. Ultimately, tier lists are decided by matchup spreads, and special emphasis is put on meta-relevant matchups. Losing to a common top tier character is significantly worse than losing to a niche low tier character, for instance. Olimar (a top tier character) potentially losing to Corrin (a low-mid tier character, probably) is not nearly as bad as losing to, say, Lucina or Cloud (top or high tier characters). How hard one loses matters too, and that will be my next point of discussions.
MU charts are generally divided into different groups. The traditional fighting game MU charts generally go with 50-50, 60-40, 70-30, 80-20, which some Smash players use, but many instead opt for 50-50- 55-45, 60-40, 65-35. Alternatively, people may use +1, +2, +3, +4, with +1 meaning 55-45 or 60-40, +2 60-40 or 70-30, +3 65-35 or 80-20, +4 70-30 or 90-10. I +1/2/3/4 causes fewer discussions than trying to go with “60-40” since to some 60-40 is +2 (many Smash players) while to others it’s +1 (many traditional fighting game players, some Smash players as well).
What do they mean, then? +1 roughly means “slight advantage”, +2 “advantage”, +3 “large or huge advantage”, +4 “near unwinnable”. Top tiers and high tiers generally only have -1 MUs at worst, although in Ultimate they might have one or two -2 as well, potentially (larger roster and more balanced than previous Smash iterations).
A -1 MU is annoying for sure and can even be seen as hard, but it’s very doable and you don’t strictly need to use a counter-pick to win (although going from a -1 MU to a +1 MU can be pretty beneficial if you can pull this off). Some Smash 4 examples (Ultimate MUs are less agreed upon, so I’m using Smash 4 examples) are Corrin versus Sheik or Zero Suit Samus versus Diddy Kong.
-2 MUs are quite a bit worse, at this point it’s going to be an uphill battle, although it is certainly still winnable. Some characters with significant flaws might have several -2 MUs (such as Richter and Simon Belmont), while more balanced top and high tiers are likely to only have one or two or perhaps none at all (I don’t think Lucina or Marth have any, for instance). Some Smash 4 examples are Ryu versus Rosalina, Mario versus Sonic, Ness versus Rosalina, and Ness versus Corrin. Doable, but very hard.
Next, we have -3 MUs, and for these MUs you generally want to switch off to a different character if you can. Having even a single -3 MU makes your character not solo viable and probably better as a counter-pick character. Some Smash 4 examples of potential -3 MUs: Rosalina vs Bowser and Donkey Kong (she wins), Zero Suit Samus vs Bowser and Donkey Kong (she wins), Ganondorf vs Bayonetta (she wins), King Dedede vs Mega Man (Mega Man wins). I’m not convinced there are any -3 MUs in Ultimate yet, but there might be. As for -4 MUs, Smash 4 didn’t have any but Brawl and Smash 64 did, I highly doubt Ultimate has any though.
What does this all mean for the metagame? Well… I’ve seen some people speculate that we’ll see a counter-pick meta, and while that’s a bit more likely than in Smash 4 (since even top tiers in this game are likely to have a few -1 MUs) I don’t think it’s going to happen too much. Ultimately, -1 MUs are very doable, and most of the top players are going to be playing top tiers and high tiers, meaning that they don’t really need a secondary. Mid tier and low tier mains benefit more from secondaries, but many of them play mid and low tiers because they enjoy those characters and/or want to prove themselves with them, so that lessens the likelihood of them counter-picking. Having a secondary that’s easy to keep fresh can be beneficial (since swinging a -1 MU to +1 does help a lot), but I don’t think it’s going to be necessary if you main a top or high tier.