I think the most significant slides down the tier list generally come from gimmick characters whose player bases aren't successfully labbing enough to keep up in the information war.
I'm defining "gimmick character" as someone who relies on weird and deceptive tricks to win consistently. All characters can play mindgames or do a one-off strategy that shouldn't normally work, but gimmick characters rely heavily on these and need to have a huge continuous bag of evolving tricks so that even if the opponent knows how to defeat everything the opponent does, they can at least keep the opponent guessing as to what's coming next. This is also a distinct idea from honest/dishonest, as that has more to do with risk/reward ratios.
If I were to be a bit broad, I'd define gimmick characters as the following:
and maybe a few more?
Many of them have trapping shenanigans or odd properties, such as Palutena's invincible bair and dash attack, mitigated by some sort of glaring flaws, often frame data. In general, they tend to fall in the mid to low tiers because fundamentals benefit them less. The ones who have done the best at keeping their character afloat is the Duck Hunt crew. Pac-Man's been viewed as a relatively weak character, but his player base also constantly tries to innovate. Bowser Jr.'s players, whether by fault of the character or the players, don't seem to be pushing as far.
While all the gimmicks in the world might not make a character high tier, the refining and development of new ones at least gives them room to grow and to keep up, as opposed to fall into complete obscurity.