Welcome to Gaia! ::

While both are possible in cartoons I think I would have to say you both may be right. They tend to look nothing like a human and most mutations tend to have at least one thing that shows what they where. (The one you show in your picture looks more like a Twinkie then a human which tends to favor your brother idea of it being a candy person.) However you may also be right as well it could be a mutation as they did not look that bad even in the finn and jake one where they turned to zombies. Maybe they are mutated candy people?
of course they could also have something to do with the lich as there is some scenes in both the one you speak of and the finn and jake versions that look and seem alike.