This one is pretty funny imo.
Unexpected reply paired with making fun of the army.
It wasn’t the fact that he has a pair of feet sticking out of this head that was the problem for the army, it was that they where flat and he couldn’t march on them.
Probably also resonated a little bit more with people in 1988 since many had directly experienced the Vietnam War draft.
Not having to go to Vietnam was a big relief for a lot of people.

Some of them are just kinda silly. Some of them are actually really clever. All of them are weird and unpredictable.