> The point of calling someone a lunatic is to undermine their opinion.

I don't see that as the point. I think they were just opining. There doesn't seem to be an argument they are making or refuting so this cannot, by definition, be an "argument ad hominem" (argument to the person).

The parent comment to yours nails it with the given examples. The first is the conclusion to arguments -- which they did not expound on -- "this opinion is lunacy"; and the second is specific argument that "you are a lunatic" and conclusion that therefore "your opinion is lunacy". That is an argument to the person.

(Not to speak to the quality of OP's comment for this message board. A comment explaining why they think it's crazy would be preferred.)