An Ad Hominem attack is misdirection. It's saying the war in Iraq was good because some reporter who said it was bad is a dick. Ad Hominem attacks are Non Sequiturs. It is not an Ad Hominem attack to suggest a reporter might not a credible source because of their history of being a dick.

Bringing motivations or character into consideration is relevant to gauging the reliability of a cited witness. The consideration of their reputation is not an attack on the subject matter, it is bringing into question whether they should be considered a reliable witness.