Even if it’s not an attack of their argument, and is stated simply as a fact, a personal attack does still work to discredit the opponent to any audience, and can therefore be considered a fallacious ad hominem tactic.
It’s basically poisoning the well. Even though you’re not explicitly saying it, the audience will infer that someone who can justifiably be described as “shit for brains” should not be trusted on the relevant topic. Even someone profoundly stupid can be right, and even someone incredibly intelligent can be wrong.
That being said, even if someone has been viciously personally attacked, if the attacker has otherwise proved their argument wrong, that’s what truly matters. It does definitely make me think less of someone if they constantly personally attack their opponent, though.
