Not It's not definitely not acceptable, but it's not malpractice, since this wasn't the provision of legal services, it was imprudent document delivery. It may also have been a violation of your privacy rights and a breach of the lawyer's fiduciary duty, but in a relatively minor way, and more of a faux pas than something to take legal action over, especially if the lawyer at least sent the documents in a sealed envelope and no one actually saw anything.
Had you fired the lawyer, was that why you were requesting your file? That might help explain, although it certainly wouldn't excuse, why the lawyer would do something so dumb and indiscreet.
I don't think the State Bar would consider this worthy of an investigation, as they are mostly concerned with serious abuses like embezzlement of clients' funds, but you could still submit a complaint to them and see what they think. Please see the link below.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.