Lemme guess Bass is "bottom tier" because he's slow right? I need to 1PP those imbeciles into oblivion lol. As for the "low tier" point, and very few characters are what would be considered "low tier", I think it's more a case of playing characters with fewer tools like Ein or Eliot forcing you to make use of everything.
The reason why I said that is because you learn how to handle a character with limited options. It makes you think smarter about decisions you have to make because of their bad matchups and sub-par toolkit. You really have to think in order to win instead of, let's say, spamming Gen Fu's 3PP and 33P.
However, I also agree with Force that the tier mindset can discourage players to learn certain characters. For example, one refuses to learn Eliot because he is considered low tier.
That may even create a vicious circle: People think Eliot is low tier --> People refuse to learn Eliot because he is considered low tier --> Eliot's potential remains largely underdeveloped --> Eliot is still considered low tier --> People refuse to pick him up --> etc.
What I mean by this is that it's hard to break people's mindsets of the potential of a character.
The thing about GenFu is he has a fairly limited moveset too, and once you learn to deal with 3PP (which isn't very hard to deal with compared to other "mix-ups"), you have to be mindful of how you use him against a competent opponent. It's just that most people don't know how to fight GenFu. But yeah, someone like Ein is great for practising fundamentals.
Yeah execution wise its almost non existent aside from multiple WRK combos, but he strives on fundamentals. Ein and Hitomi are perfect for learning DOA easy to pick up but hard to fully master they are DOA personified.
"arguably the easiest". Doesn't mean he's the only easy character. Point is that he's pretty darn easy to use all things considered. Though very few characters in DOA are genuinely "difficult" to use.