I don't know if he can. But the inability to answer a question does not make it a dumb question. Someone may be able to answer it. Or maybe no one can. Either way, I think it's a question we should ask. Do we need the DHS or is the system we had before good enough?
It is a dumb question. It's asking him to, at worst, predict the future, and at best, be fully aware of the department's subjective impact on its own roles without ever having worked in that department. If the person asking was really interested in the DHS' impact, they probably should have referred to a specific role it plays and how that function has or hasn't changed since its inception. Asking a vague question like "ARE WE SAFER?!" makes no sense. It's the difference between asking if Border Patrol makes the country more or less safe... or asking if the Border Patrol statistically lowers the amount of illegal immigrants. One is a vague, open ended question that is just conjecture, the other is a real topic.
It is a vague question, no doubt. But that's their jobs: to take all the various question and impacts and condense it into a Yes or no vote. "Are we safer" is just a short form of "Does the Department of Homeland Security do the tasks they are assigned better than the various departments acting as they did before." It's a question he should be able to answer, especially as someone who needs to vote on justifying how much (if any) money should be spent on it.
And since be should have access to all the various attacks and plots that the DHS has foiled over the years, he should be able to answer the question with some accuracy.