I think it depends a bit on whether you consider tiers to be relative or not. I think that tiers should have some criteria, and if a character reaches those criteria then that character belongs to a certain tier. Imagine if the worst character in the game goes even with every character but two, who that character slightly loses to. I would be okay with calling that character "Mid tier" instead of "Low tier" or "Bottom tier", even if it's the worst character in the game.
Take a game like Rivals of Aether, for instance. That game has 13 characters and is considered fairly balanced, and the community seems split on a tier list. Should the bottom 2 or 3 or so be considered "Bottom tier" or even "Low tier" if they're not considered to be much worse than most other characters?
Another issue I have with the term "Bottom tier" is that it's not wildely used. "Low tier" is commonly used and people talk about Low tier tournament (not Bottom tier tournaments), they make videos on "Why you should main a low tier" (not "Why you should main a bottom tier"), and many people call characters like Jigglypuff and King Dedede "Low tier".
TV Tropes has a page for Character Tiers, and here's what that page says about Low Tier and Bottom Tier:
- "Low Tier: You probably don't want to choose these. They could theoretically be useful, but choosing such a character is a suboptimal choice; take only if you need to fill space. Sometimes, these characters find a niche for their shock value, or because they work well against unprepared or surprised opponents. This sort of usage stops working once your opponents get wise, at which point you should return to a higher tier.
- Bottom Tier: Joke Characters, and those who are just bad. They may have an advantage over someone in top tier, but outside of that specific situation, be prepared to have extreme difficulty using a Bottom Tier character in high-level play."
The worst characters in Smash 4 seem to fit "Low Tier" better than "Bottom Tier". It's also worth noting that in the Reddit tier lists, the worst characters in the game are generally ranked C- instead of D-tier (even though D-tier is an option).
On another subject, I think we should skip rating Miis this time. It's pretty clear that most people tend to underestimate Miis a bit (at least with all sizes allowed and customs on), and 1111 Miis are pretty bad anyway and hard to rank since almost no one plays them.