I'll just post this again because I can't do a better job explaining it than Vox did.
https://www.vox.com/2015/10/28/9633420/ted-cruz-republican-debate
The moderators did NOT call Trump a comic book villain. The question started like this: "Mr. Trump, you have done very well in this campaign so far by promising to build another wall and make another country pay for it. Send 11 million people out of the country. Cut taxes $10 trillion without increasing the deficit." and ended by asking why his policies sound like "a comic book version of a presidential campaign."
That question is tough. But it is NOT an ad hominem. The reality is that those proposals, as accurately stated, are outlandish. This is poe's law territory, if you were going to write a ridiculous policy platform that's pie in the sky it would be hard to top that one. John Kasich was the only person on stage willing to say as much and the moderators were attacked for effectively holding a candidate's feet to the fire. Paradoxically, that's exactly what we usually ask journalists in these situations to do.
The full text of the question that was framed as "Can Ben Carson do math?" is here:
That is a substantive question. What is the fair way to say "your math doesn't add up, please explain?"