Yeah, sounds like you're misunderstanding what tiers are. The vast majority of the time, the developers don't purposefully make some characters better than others. It just happens naturally because the characters aren't all identical, and so some end up better than others. Tiers are just simplifications of which characters happen to be good and happen to be bad.
And it's not like characters just beat those on lower tiers. It's very possible for a bottom tier character to have an advantage over the top tier character. It's just that the top tier is overall at an advantage against all matchups. An easy example is Honda in Super Turbo. He's absolutely garbage against anyone with a fireball, but if you don't have one you get wrecked. The majority of his matchups are 3/7 or 7/3. I want to say Honda vs Cammy is 11/-1 'cause it's pretty much impossible to lose unless the Honda is really messing up. Meanwhile, OKen is like 1/9 for Honda and makes him cry. Where does he place? Solidly mid tier. Meanwhile, Dhalsim has pretty much all great matchups, so he's top tier. That's how tiers normally work.
So really, picking high tiers isn't gonna secure wins and it's a really simplistic way of evaluating characters. What you want to really look at is the individual matchup evaluations. Ideally, even (or near even) matchups for everyone against everyone else would be amazing, assuming the characters aren't carbon copies of each other. But that's pretty much impossible. Still, DOA isn't unbalanced that low tier characters can't win. It's pretty much always possible 'cause of the way DOA works.
And as for 2D games being more tier dependent than 3D, that's not true. Especially for that reason. I mean, just think about it. Pretty much always, Aoi is gonna take more damage from a juggle than Taka. Not that different from hp values at all. And it's not like all 2D fighters have different amounts of health for different characters. As for movement... Well, c'mon. Unless you're thinking in terms of old school 2D games, that's far from the truth. Airdash, super jump, fastfall, fly, triangle dash, homing. I don't recall seeing any of those in a 3D fighter. And Devil Jin's fly stance doesn't count. It's true that 3D games have the option of going in the foreground and background, but it's a lot less intensive in terms of the vertical spacing. Majority of the time, it's range that's talked about when we're discussing hitboxes for 3D games. When we're talking 2D, far more important is the vector (or angle, rather). The amount of control some 2D games allow in the air lets characters
space on
two axises. Think of it as manually crushing. It's a pretty big blanket statement to just say one type of game is more restrictive than the other when, in reality, they both have unique aspects to them that allow freedom in different areas.
Also, nerf Gen Fu.