Is it appropriate to contact a journal about grammatically erroneous editing?
I can't give you a yes / no answer, but I can say what's likely to happen if you choose to complain.

If you choose to complain to the publisher: chances are they'll ask for some examples of poor copyediting and form their own opinion. If they agree with you, they might change the copyeditor (if it's outsourced) or provide him or her with the negative review and pointers (if it's an in-house employee). If they don't agree with you then probably nothing will happen other than giving you a vague "we are aware of the issue" response.

If you choose to complain to a member of the editorial board then:


It's possible the editor will say "this is none of my business, approach the publisher", or forward to the publisher "I received this complaint, what do you think" (in which case, see the second paragraph).
It's also possible (but in my opinion less likely) that the editor will take things seriously, in which case he or she will contact the publisher. Again, see second paragraph, except this time the publisher is also dealing with an unhappy editorial board member and so would be more inclined to make changes.


Finally if you choose to just make your corrections then your corrections get implemented and everyone just moves on to the next paper. 

Having said all that, I gotta say, I don't see any difference between "This can be proved via the method of Gauss, see [1]" and "This can be proved via the method of Gauss; see [1]". Both simply say that the reader can find the proof in reference 1. It looks to me like an unnecessary correction, but it isn't per se wrong. I checked with another editor and he found both OK too. If you can confidently say that one of them is wrong, you have a level of English mastery that most other native speakers aren't even aware of.