She IS a criminal. She broke the law and her work places rules. She happily abused the power and trust she was given, completely overstepping them too. All to mess with someone for her own entertainment. She's a complete PoS, and without this blowback she would've not thought twice about it.
Remember, she was the one who set the MC's girlfriend up with someone else. She was the one looking down on adventurers and stuck in a very outdated mindset. She was the one doing a bunch of nasty stuff, including iirc covering for those other assholes because they had connections.
At the very most, she abused her employment position, and that led to a disciplinary dismissal. The reason her bosses called her a criminal, was because of what her actions are going to result for Japan as a whole. They're not saying she literally broke the law, because she hasn't. What's weird is that Katsuki wasn't shit canned as well. He also abused his position, used the restricted terminal for his own personal business. That's equally as bad as her. And unlike her, who says she's not smart, he should be smart enough to realize how big a transgression he's committed. And yet, he only got demoted and forced back 3-5 years in his career progression.
Unless you're going off of information in the WN, the manga doesn't show her doing any of this for entertainment (And in this version she might have totally different motivations than in the WN). Shimizu Mizuki has only shown up in 2 chapters so far, 2 and 9. From what we've been shown so far, she didn't set up MC's ex with another employee for entertainment, she can honestly believe that the Higher level employee is a better boyfriend, than the nobody MC. And the manga doesn't show her covering for Nomura either. Saruwatari and the male employee are the only ones that make an appearance in chapter 5. The unknown Male employee is the one that implies he knows about covering stuff up, not Saruwatari.
Being disrespectful isn't against the law. It makes her a bad employee, which again, isn't against the law. And it's not like she forced Kaori to date him. She personally made a shitty choice to cheat.
Ripe old age of 24. Chapter 4, and coincidentally Page 24.