She's not too powerful though. She takes a lot of skill to use and a lot of practice, and not just anyone can win with her.
"Too powerful" would be Meta Knight in Brawl where any dunce could just pick him up and win with him because the rest of the cast couldn't punish.
The first paragraph could apply to Fox in Melee.
If Melee was patchable I'd nerf his furry butt instantly without a second thought.
A character taking skill shouldn't be relevant to balance. Same with a character being easier or harder.
Metaknight actually takes some skill to actually play at a top level, same with fox in Melee. But as you agree balance would nerf both is possible.
Some players think Rosa is too strong, I have no opinion at this time, do you think she isn't a top tier character? If she is, that is why some people think she might need toning, like she has been in every patch.
Now, too tier =/= doesn't mean must nerf and we okay musical chairs in the top tier seat like WoW does with classes. Only if in perspective with the cast if she is too strong.
Shiek fits this in smash 4, IMO, where she is a tad too strong. Rosa I'm not as sure. My opinion though.
Some players like Zero have said she needs it though.