This corset hate must be related to tightlacing, more than correct corset wear by itself...
I mean, tightlacing is the general association people have whenever there's any mention of corsets, contributed heavily by Western historical novels, even classics (for example: Gone with the wind).
I find the fear of the FL in line with the knowledge held by most people. Who knows maybe in the setting of this novel tightlacing is actually a thing, so her fear was actually reinforced by the world inside the novel instead of correcting it. Which would make sense since she "wrote" it and projected her own fear in the setting itself.
Let's not forget that she did not actually travel back in time, but actually travelled to a work of fiction written by a very young (so possibly not very informed) author.
The premise is not historically accurate, ergo, her fear is justified because in the world of the novel, corsets do cause the problems she keeps obsessing over.
If you wear corsets, that's your choice, modern corsets are both comfortable and beautiful. Generally corsets in the past were also fine, the problem ensued when people started taking things to the extreme. But that does not obligate an author into being historically accurate in a story set in a fantasy world imagined by an imaginary character.