They are mocking and ridiculing Trump for refusing to answer James's questions, suggesting that only guilty people take the Fifth.

In actuality, Trump was smart to invoke the Fifth. Moreover, the entire episode helps to remind us of what our American ancestors did for us with their insistence on a Bill of Rights after the Constitution was enacted.

One of the reasons that anti-Trump supporters are so hopping mad over Trump's taking of the Fifth is what lawyers call a "perjury trap." The idea is to get a person to talk and talk and talk, in the hope of then finding places where he didn't tell the truth, and then charge him with perjury. In fact, oftentimes the person isn't even charged with the substantive offense for which he was being investigated. He's only charged with lying about certain aspects of the situation while he was talking or testifying. 

You will recall that that's precisely what happened to the famous television personality Martha Stewart. She was being investigated for supposed stock-market violations involving insider trading. When the FBI came to interview her, she unwisely decided not to take the Fifth. She was ultimately convicted  — not of insider trading but rather of lying to a federal agent.

Oftentimes, prosecutors subpoena targeted individuals to give testimony because they lack sufficient evidence to charge them with a crime. They hope that the individual will provide them with the evidence they need to convict or, alternatively, with the opportunity to charge the individual with perjury.