If that was the case, Fox and Falco would be at the very bottom of Melee's tier-list, haha.
Tier-lists just show which characters have the best chance of winning a tournament in the current perceived meta. As new technology is found, players become better, and people become more aware of what every character in the game is capable of, the tier-list shifts and changes. Oftentimes dramatically (Anyone remember when DeDeDe and DK were top 6 and top 10 in Brawl respectively?).
In Smash 4, because of it's better balancing compared to past titles, a character being low-tier doesn't necessarily mean they're complete trash and totally unusable; they're just worse than most of the cast. A lot of them still have niche uses and can compete in some fashion. For example, a little while back Trela (notable Robin/Shulk main) won a tournament with Mii Swordfighter and Nairo's Zelda has taken sets off Nietono's Sheik.
It's gonna be interesting to see how much the new patch shakes up Japan's tier-list. Based on how terribly they overreacted to Greninja's nerfs, I fully expect Diddy to drop to, like, the middle of B-tier and S-tier probably won't exist anymore.
For those who didn't see Japan's last tier-list, and I'm sure that's most of you because IIRC it was never posted here: