Yes, it is acceptable behavior for Mrs. Clinton to state that she did not want anyone to view the private emails mixed in with her State Department emails. Private emails should be her business and hers alone. Her public service is only one aspect of her life. She is also a private citizen and has a right to personal private emails.
No, the manner in which Hillary Clinton handled her email before and after she was at the State Department was unacceptable. She was told the laws about handling emails and turning them over both when she started as Secretary of State and when she left, yet she still decided to disobey the law and handle her emails in a manner that is illegal.
The department system issued numerous warnings dating back a decade about the cybersecurity risks of using personal emails accounts for government business. Mrs. Clinton was personally sent a memo in 2011 warnings of hackers trying to target unclassified, personal email accounts. She was also given a classified, in-person briefing on the dangers.
Any adult can go to Gmail or Hotmail and can make their own e-mail account. By having personal e-mails forwarded to a private work account, Clinton opened herself up to scrutiny. If she had e-mails that were truly private, she could have set up an e-mail account for private use.