Uncomfortable with calling out an editor.

If I read this correctly, you merely assumed she was referring to you when you read "middle grade horror". You may not have been her only client at the time, you know.

And if I'm correct, you marketed the book as such when you published it, so that may be a reason why it received bad reviews. Secondly, you never had a beta reader? You never researched--even out of curiosity-- what market you were writing for or how to self edit with a plethora of information literally at your fingertips?

Also, as others have mentioned, there are different types of edits. If the story had structural problems but you paid for a copy edit, then you're in bad shape. A reputable publisher would not hire an editor without good credentials. Something got lost in translation when hiring her. I'm curious to see the string of emails...

Don't call her out with no proof when it seems that you may not have researched or planned properly. Whether she is at fault or the fault is yours, chalk it up to a rookie mistake and be more cautious. Educate yourself. There are TONS of resources available on the internet. Never go into something blindly.

I'd also add that you shouldn't really worry about your debut novel being a flop. That's not the type of thing that really matters; sure, if it sold really, really well, it might make it easier, but publishers and agents understand that people get better at writing as time goes on, and that they learn from their mistakes.