What's the opposite of eye candy?

Eyesore? Affront to all that is holy? Mirror-cracker?

"Eye Candy" is most often applied to people. In that case, the opposite is "Double Bagger".

Paraphrasing the Urban Dictionary entry (vulgar) -- which the term predates by up to 60 years:

Double Bagger:
A (person) so ugly that (making love to) (him or her) is only possibly(sic) with the use of two bags: One over (his/her) head, and a second bag over your own head in case (his/her) bag falls off.

In the general case, "Eyesore" is common, as idunno answered.
But I prefer: "A blight for sore eyes" (^_^)

The term "eyesore" is well established. It is usually applied to buildings, but it could be used do describe pretty much anything that is unpleasant to look at.

That old office block is a real eyesore.

Like many terms, this could have many opposites, depending on the sense you have in mind.

"Eye candy" can simply mean a person or object that is pretty, or it can mean a person or object that serves no purpose other than to be pretty.

In the first case, "opposite" could mean someone who is not particularly pretty or attention getting, e.g. "plain". Or it could mean someone incredibly ugly.

In the second case, "opposite" could again mean something ugly. Or it could mean something that is pretty but also serves a useful purpose. Or it could mean something that is not pretty but instead serves a useful purpose.