Yes, violating the rules regarding government correspondence is a serious matter.  The fact that she asserts that her server was not a serious matter teaches that she has concluded that she is above the law.  For a leader to conclude that they are above the law is a very very serious matter.  It is interesting that out of 300 million-odd citizens the democrats have chosen the wife of a sleazy sexual predator ex-President as the best of the best to represent them.