The editor for my book was a not all that useful. She was late on editorial letters, haphazard in her comments, and as far as I can tell really didn't do that much.
I turned in the acknowledgements section of my book and said thank you to her, but my agent is saying it sounds too terse. Do I really have to say something nice about her?
There are ways to acknowledge effort without saying thank you if this is something you really want to do.
Thank the editor for acquiring the book, or if she inherited it when the original editor left, thank her for stepping up when the job needed doing. Acknowledge her contributions made the book better if you think they did, even in a small way. Acknowledge the time she spent on your book even if you didn't think it was enough.
It's never wrong to be generous in these matters. You don't know what was going on with her during the editing process. A terse thank you is better than silence (I've had a few of those) but a little butter on the toast is a good thing.