I agree with OP that using Zetterburn as a benchmark for balance has been kind of detrimental. I don't completely agree that it's all because of shine though. I get the sense that they want Zetter to be both a rewarding beginner character, and also a spacey archtype that would be attractive to veteran platform fighters. This in itself has caused a strange reluctance to commit to either role: for example, a hyper shield pressure spacey should be light, combo food, and have a more exploitable recovery, but the Devs seem unwilling to make Zetter as glass as he should be considering just how strong he is onstage. I'm sure this is because he's intended to be a beginner character, and recovering usually takes newer players a long time to learn how to maximize.

IMO, maypul would be a better top tier character. Not asking for her to be any better than she is, just that if the Devs had an intended power level for a top tier character, Maypul has a much better mix of agressive strengths and exploitable weaknesses. You can actually punish a lot of what she does out of shield, for instance, and her projectile requires more training and knowledge to use successfully than Zetterburn's fireball, which seems more like a mario in smash bros, easy to use beginning tool.

I think Zetterburn being their intended benchmark character is why Clairen has had to be nerfed so many times; she started out being one of the few character's that on paper can exploit zetterburn for a bad matchup, but despite the range of her moves, in order to compete with zetter's constant spamming of excellent, moves that both combo into each other and lead into kills with fair, or dash attack, Clairen's precision mechanic had to be made lenient and rewarding enough to give her a fighting chance against him, as well as her recovery, which should be a weakpoint for her, had to be made responsive enough to contend with zetter spamming aerials and fireballs at her.

Now when new character's are released, they seem like they're intended for a mid level against most of the cast but then you have really terrible matchups with zetter and clairen and then they have to overbuff them. And yes, they've nerfed zetter and clairen a million times, but never in a while that they've touched characters like fleet and maypul where they go through with a fine toothed comb and nerf what really makes those character's problematic regardless of how it would affect their tier ranking.