Same can be said for sarcasm. Its why we have <sarcasm> tags. However the entire content of the post is such that it cannot make sense but to be sarcastic/ironic.
I wouldn't sweat it, people downvote when they suspect something might be humorous, sarcastic, or a parody. Adding a tag like <sarcasm> or <humor> simply confirms it for them so they can downvote with a clear conscious.