In my view, it's unethical to portray oneself as a technical writer if one is not able to write even a couple of sentences without making grammar and punctuation errors.
If one writes technical material and gets paid for it, then one is in fact being perfectly honest in describing oneself as a professional technical writer. Now, if one were to say one were a competent professional technical writer, that would be another matter . . .
Simone makes a valid point, though. From your post, it seems as if you need to fix your grammar generally. Also, do you have many British clients who do not comment on your grammar? Or are most of your clients American? If the latter is the case, then that would tend to indicate the problem is with the writing and not the clients.