If multiple people are pointing it out, it is something you’ll want to take more seriously than you would if simply one person pointed it out. It means that readers are getting pulled out of the story enough to comment on it and while in the end it is absolutely your story, I assume you post your work because you want people to love it and become immersed by it.
So, first question is yes. I have absolutely dumped books when it opens up with the character getting ready and looking at themselves in the mirror in order for the character to be described. I feel bad for saying this, but it is the most common reason for me to leave a book before the first chapter is even finished being read.
I have seen this done well, however, but it’s pretty rare.
A mirror can be used to show a subtle hint, such as a scar or a strange hairline or what have you, but when it’s used to give the reader a lowdown of what the character looks like really quick, it is a big turn off for many. It’s one of those scene that doesn’t add to the story and rather pauses it to give the reader information. Description should blend it and be virtually unnoticeable, which can be a tough thing to accomplish.
Second question is also yes and no. Revising scenes is a part of the editing process. This is just a first draft and when that first draft is finished, there is still a lot of work to be done. I have rewritten scenes and chapters many times during editing. It’s what we need to do in order to turn our books into what we imagined in our minds rather than what we just perceived them to be.
But no, an editor cannot do this for you. An editor points out mistakes and improves sentence structure. They take out unneeded words and change repetitive words. They let you know when your punctuation is off or when you’re using too many dialogue tags. As soon as they rewrite a scene for you, they become a co-author. It’s your book and the best way to evolve in writing is to find out the weaknesses and work to improve them.
And no, it isn’t bad to rewrite a big part of a story. I once went to a writers conference and the guest author, who’s books are famous and one that was turned into a brilliant movie, talked about first drafts. She’ll send them to her critic and they’ll send it back saying the first 1/4th or 1/2 of the book is great, but the rest has to be rewritten.
No published book you love is a first draft. Most of them are a fifth or sixth or seventh. Most required extensive rewriting and were torn to shreds in order to become something great.
But this can all be saved until your first draft is done because I will tell you that most of us do not keep our original first chapter. That first draft is to help you get the story onto the page and get to know all your character’s on a deeper level so you can best figure out how you want to present them to the reader.