This is going to be fun! My MIL has a friend who complains about everythng. She is so good at making people miserable, she's wound up getting things free or paying next to nothing just so she'd leave the poor customer service people alone. This is a very true story. The poor woman even looks like she complains about everything.
On that note, it takes all kinds to make the world go around. I haven't read the other posts, but I gather that she was pretty detailed in her message to you about what she thought was wrong with your work. I haven't seen your work and so I can keep an open mind about this. I also don't have a business yet baking cakes to sell so I'll be talking from a different standpoint that most people.
What I would do if I were in this situation (planning ahead) is to ask her to give you examples (by providing pictures) of people in your area who do the same kind of work you do (make her work for it here) so that you have an opportunity to compare your work with theirs. Also ask for pictures of the cake from the event (so you can see if something happened after delivery/pick up, etc ). Once you have the examples of other work and your work next to it, see how your quality compares and decide from there if YOU feel YOU met her expectations.
Now, if there isn't anyone else in your area who does this type of work to the quality that you do it, you will know because you'll have proof and can pat yourself on the back for that, and you win, and nothing further needs to be done.
If she proves that there is someone else who gives better quality, you know that you are going to have to practice more to compete with them. In this instance, you have an opportunity to offer a discount on her next cake (a percentage not a specific dollar amount), or a refund of that same % on the cake you made for her that she is not happy with (but it's your choice, don't let her choose between the two).
An unhappy customer tells more people about it than one who is happy and we all know the customer isn't always right, and it takes all kinds to make the world go around.
I have worked in a customer service field for many years. I owned my own transcription business and worked with office managers and doctors who thought they were better and knew more than I did. You learn to smile through your clinched teeth. Let them eat cake will take on a whole new meaning for you.
I don't know anything about you or your work, but my gut tells me that you are serious, dedicated, creative and very driven. It's up to you whether or not you let them take that away from you. Just because people say something, doesn't make it true unless you believe it to be, and you have 99 other people who have never said anything about being unhappy.
And, if you want to be catty about the whole thing, if you offer the % off her next order, you can always be too busy for a while to make a cake for her until you feel like it would be okay. Now if she comes back that time with a problem, you'll know she has a bigger problem than your cakes. At that point, you don't have to try to make it better for her, you can have the satisfaction of not taking orders from her any more. and you can tell her that you think, since she is so disappointed, it would just be best if she used someone else from now on.
Life is too short to play by someone else's rules. At 44, I am finally learning that. I hope this is helpful to you. (I also hope I remember it when it's my turn.)
My answer "well my dear you are always welcome to do better." 
That's the polite way of saying "Bite me." 
