If Mike Flynn paid hackers to break into Clinton's poorly secured server, it could be a violation of the law, but it couldn't be a campaign law violation because Clinton left the State department in 2012 (well before the election). This would be well outside Mueller's charter.
If Mike Flynn was attempting to find out if hackers were successful in breaking into Clinton's poorly secured server, so as to demonstrate how irresponsible she was at handling classified information, then I have a hard time understanding what law was broken. Perhaps Mueller is attempting to slap a label of "collusion" on this, or a similar action. But this is neither illegal nor does it "de-legitimize" the election of Trump, as no such proof was ever made public.