Is it acceptable to ask the editor for feedback about a review I wrote?
This is totally acceptable. It means more work for the editor, of course, but I think most of us are also teachers and hope that by helping you become a better reviewers, that we can draw on your again in the future.

In particular, in your email, you might want to ask whether it is possible to get the other reviews. That may, strictly speaking, not be quite allowed, but I have often done that to help younger scientists see what other reviewers saw in the paper in question.