It's certainly fair to bring up the business dealings of Mezvinsky, but in the overall scheme of things he's not that important. I'd rather worry about the blatant cronyism already rampant throughout the government before worrying about something that may or may not be occurring. At this point you couldn't untangle the banking system from the political class even if you tried, anyway.
Marc Mezvinsky's business dealings are off limits in the current political campaign. Even though he is Chelsea Clinton's husband, he is not running for office, and therefore, his business life is not relevant. A family member does not represent the candidate. The only way it would be relevant is if Hillary Clinton was directly involved.
The presidential candidates do not choose spouses for their children. These spouses do not have anything to do with whether or not a person is suitable to hold the office of president. The business dealings of Chelsea Clinton's husband, Marc Mezvinsky, should not be used against Hillary Clinton or even be part of the political campaign.
Marc Mezvinsky, as an individual, is open to as much criticism as any other businessperson. He can and should be evaluated for his work on his own merits. However, his individual business dealings are not relevant if they do not directly involve the person actually running for political office. If he makes exploitative or illegal deals, then he should face punishment for that, but those hypothetical deals would not be relevant to Hillary Clinton unless she was actively involved in those business decisions.