Holy mackerel is an expression of surprise, astonishment, joy, glee, incrudulity.
Holy mackerel is an ejaculatory term - meaning it occurs rather sudden.
The meaning of the name is an oxymoron, as the mackerel (Atlantic mackerel) is one of the smellier fish and juxtaposed against the Holy, there is nothing Holy about that smell. So in other words its a bit ironic and thus is funny to hear.
Holy Mackerel, Batman is a fav culture classic from the 1960's TV series show Batman that starred Adam West and Burt Ward.
Holy Mackerel Batman !